This really sort of works in real life, too. You can, in fact, bum money off your friends. If you keep doing it, they may eventually stop being your friends, though.
Not that this means, having such an exploit in the game is okay, yes? Or is there really a hit in relations in-game if you do it? Then it could be sorta fine. Only, the penalties for such abusive trades seem to be a bit... on the light side (meaning, I haven't noticed any, short of making the other empires envious of you, the more you outstrip them).