The laws and licensing issues governing copyright and trademark for and or by the US government are a quagmire of wording and exceptions. This is an area that changes pretty regularly and keeps most copyright and trademark lawyers in full time salaries trying to decipher and keep up with what is happening.
Government works are public domain. Works produced by contractors under government contracts (or submitted in anticipation of such contracts) are protected and restricted under U.S. copyright law. The holdership of the copyright depends on the terms of the contract and the type of work undertaken. The end run around public domain is that the government or the DoD in this case will allow contractors to protect technologies etc despite it being done with taxpayer monies because they work in a clause in the contract to allow the government to share that technology as needed under the Federal Aquisition Regulation Act. i.e. a missile defense system on a fighter jet may be used for commercial airliners if the DoD so chooses. They have license of usage but must have the contractor agent permission first This is usually a contractual stipulation up front). Conversely this works both ways and the contractor has usage rights as well and if they see a possible market for that missile defense system they can seek permission from the govt. contracting officer to offer that technology up for sale to those airlines. Several Defense Acquisition Regulations System clauses protect and recognize the contractor's copyright, while they direct the contractor to assign the copyright to the Government. However, the same clause defers that a contractor "retains use and disclosure rights" even after such an assignment
A relatively old but recently more active player to the game is the US Army Institute of Heraldry. They regulate and control the various unit insignias and emblems and these are protected under Title 18 trademark regulations. The reason they have become more active is because of the current popularity and "selling" of the military. Our soldiers are heros and are quite the celebrities these days. (understand I am not bashing the military here) and not since WW2 has such positive and expanded exposure been given to the troops. This has created a market that can be exploited. So naturally the army et al, wants its image monitored and controlled. Hence the reason they are more protective of their properties. The tamiya deal was more or less more for trademark licensing usage not copyright. In that the shape and image of the item is clearly recognizable as a property of that company. i.e. Boeing makes the B17. Martin makes the B26, they are clearly recognizable corporate designs and properties despite that they were made for the US government. Newer properties like F14, f16's etc are still viable properties and have modern ideologies and modern corprate paradigms attached to them so they are controlled more than older properties that are long out of inventory. But I think if you look, don't alot of the box art labels show B17 not Boeing B17? AN end run around licensing and trademark. Simple nuances like this allow companies like Andrea to end run around licensing protection with figures like Science officer (Mr. Spock from ST) Web Slinger (Spiderman) Outlaw (Robin Hood) etc.
Look at it from the standpoint of the owner. My design, my work my product, I should receive every benefit possible from that work. Hence trademark and copyright laws. In modern terms these are questions and answers that are measured in nuances and interpretation. Kind of like contract terminology, "must have" and "will have" are two entirey different phrases but to an onlooker may mean the same thing. Sort of like the definition of "is"
Pursuit, enforcement, circumvention of the laws and protections are a matter of convenience, resources and conscience. Will the DoD or their associative departments and or contractors pursue you for making your modeling products? Probobly not. You will fly so low under their radar that you are not worth the time to glance at the website. Will you impact their image? No. Possibly if you created enough of a buzz that made them take a look or negatively impacted their licensing they may get interested but I doubt it. Its driven by money. If they can make enough off of you to make it worth their while maybe, but for what they pay their lawyers, you'd have to be making more than kit money to pay for itself. Tamiya works in terms of 6 figures on a product, if not more, and global economies so for them it was worth it. And I think there was more to that than we were lead to know. My feeling is that a large company like Tamiya can afford to open that Pandora's box. Think of how much of their smaller competition would cease to operate if they all had to pay corporate licensing fees. Create and manipulate a market and you own that market.
Plausible deniability, the plethora of public domain property and subjects and the complexity of the law work in your favor. In this field I wouldn't worry too much about it.
Try and knock off Disney properties though and hired assasins will come to your house, and wipe clean your record of existance and anyone that knew you or heard of you. No two stones will stand together that may have witnessed your being. They don't mess around and will bring all guns to bear to make a point. In my business I see trademark and licensing issues pretty regularly and even when I thought I was in the clear, after talking to a lawyer, I still got some pretty intimidating letters and a visit from disney orcs after doing a job that hinted at via lampoon that included a disney character silhouette. A competitor of mine got the rude awakening of his life when sheriffs, marshals, the IRS and two large moving trucks showed up at his shop and house to confiscate everything, lock his bank records, and spent an enormous amount of time and effort to destroy him because he was making disney knock offs and selling them. They like to make an example of everyone.