How is this legal?
Assuming these trademarks have been maintained by their original owners, with the exception of the Wink / Dragon shirt (who I would think went through the proper channels to license the image from Sony, though I have to say I wouldn't be totally surprised if they didn't, considering some of the business minds involved), it isn't. (And the (awesome) Trap shirt is in a position to be smacked down by both Fremantle *and* Disney.) It's one of those things where either a) nobody's noticed or b) nobody cares enough to complain about it.
GIMME A B! B! B!
Seriously, though, without naming a name, I had a conversation with a knowledgeable (but anonymous) friend who basically laid it out for me this way: We live in a world FULL of do-it-yourself content; homemade shirt designs, YouTube videos, etc., and there's more than ever for a corporate legal department to keep track of. As a result, 99 times out of a hundred, the legal department is looking for "Is it taking money away from us?" and "Is it actively causing harm to our brand?" If the answer to both questions is "no," they probably won't touch you. That doesn't mean I consider myself in the right, and you have my solemn vow that if the C&D letter arrives tomorrow, I won't say one word of complaint. And to make myself feel better before the upload, I did a RedBubble search for "Ackbar" and "It's a trap." Even though that doesn't justify anything, I think you'd be surprised at the readily available results.