If they had it so there was a limited time, every item would have a time limit on it, which would be horrible. Having it so only people in your fireteam can trade for it is ridiculous as well, because you could just join a fireteam with a low level character in a higher level place and get them free gear. That's game breaking. And if you didn't want to trade straight away, the game would have to remember who was in your fireteam for every item drop as well, which would be hard to keep track in an mmo (where you should be getting frequent drops in a mission.)