PS3 Saved Games

I was wondering, since you can so easily transfer saved games from the Internet to the Playstation 3 (perfectly legally, by the way) wouldn't it be really easy to exploit that and get a whole bunch of trophies that you yourself didn't earn?

I have another question though, why can't you transfer some games to an external hard drive and not others? I really don't see a reason for this, some of the games that I want to transfer don't even have trophies so that can't be it...

trophies only works on the user that gets them, and some games have online score boards which disallows gamesaves to be shared.