Q1:
This question comes up now and again.
The reason has less to do with FSM or Kalmbach, and more to do with the laws in your own country. For instance, I live in British Columbia, Canada and can enter, but if I lived in Quebec, Canada, I couldn't because the Provincial law there prevents it. Why that is, I'm not too sure, but that's the way it goes.
When it comes to far off places like Australia or Europe, an additional reason is that the cost of shipping is prohibitive, and you would be subject to duties and taxes on the winnings, and that could match the cost of the prize.
Sucks, but there you go.
Q2:
This would be best left to FSM to answer, but the likely answer would be that they thought of it, but decided against it. I surmise that an aircraft only mag would necessitate an armor only mag. That would double the production costs, require twice as many articles to keep the page count similar, and would tick off modelers who don't limit themselves to one genre. For instance, I would resent having to buy two magazines, when one suffices.