As far as I read into copyright laws (not all that much), I believe it's OK to 'sell' these sorts of things as long as the vendor isn't making a profit. For example, I buy one of those shirts for 'cost-price' - in other words, the price it cost to create the item.
As I understand, it's OK to exchange and/or create 'copyrighted' good for non-profit means. In other words, selling these shirts at break-even because the UNSC logo isn't a trademark. This is why it's OK to copy music and movies as long as you don't sell them or gain a profit from it (although I do believe this practise is still frowned upon under corporate law for illegal distrubution).
Creating anything featuring a trademarked item is grounds for trademark theft - this is inexcusable and *WILL* result in a court appearance. For example, making a Halo 3 logo t-shirt will earn you a trial and a possible fine or jail sentence.
In other words, I'll mirror what Adam said. It's OK to distribute these shirts as long as there isn't money passing hands. Although I recall reading that money exchanged as 'gifts' would be acceptable..
