There would always be people looking to pirate anything and to gain from someone else's work. Unless you have an ALPS printer (which, if I'm not mistaken, are longer available) you would not be able to print a decent and totally useful decal sheet from a scanned image.
The problem is that, except for the ALPS brand printers, (and one other which I don't recall at the moment) no computer printers print white. White is essential to making a good decal. It provides opacity for certain colors that would otherwise have to be built up in so many layers that the decal would be much too thick.
Then there's the simple matter of white being a color of a marking itself, such as the star (and bars) of the US national insignia.
Also, I've seen many a decal printed with an ALPS printer, and I am not convinced that the quality is there. Nothing can hold a candle to a professionally printed decal sheet. That's just my opinion, however!
Don't get me wrong, you CAN make a fairly decent decal with a printer. It's just that, for me, "fairly decent" don't cut it!
A friend of mine printed up decals for "The Huff" in 1/32. (For non-aircraft modelers, "The Huff" was a piece of nose art on an F-86 during the Korean War that depicted a fire-breathing dragon.) I, personally, thought that it was a mediocre decal. He scaled them up from 48th markings. Certain problems inherent with the process of scaling an image up can be solved with a good imaging program, but it's the actual printing that leaves something to be desired sometimes. I admit, though, that I can be very critical of such things. Many of you may not be so "attentive", so to speak.
As regards to the copyright issues, and this may spark some debate, as long as one's intent is not to profit from copies of a decal sheet, I see no problem with making a copy for personal use. Matter of fact, someday, I'd like to be able make copies of such things as national insignia for use with decal sheets that come with none, or not enuff. Some may look at it as stealing potential profits from a company, but not everyone can afford to buy decal sheets which are now approaching $10 apiece. If it means keeping a few bucks in the wallet for "important" stuff, then I'm not opposed to committing what, in the eyes of the law, may be labeled as "petty larceny".
Slap on the cuffs and book me, Dan-O!
Fade to Black...