See every time the person will not send through the same address he has the ability to change it different address in order to do multiple transaction, how can people will identify if he keep on changing multiple address means, these thing should be take care so that we can control money laundering through bitcoin.
They can use mixers along the way and that would help a little but if you are talking big money then no mixer can handle it.
And what sort of "control" are you talking about, government laws and regulations? No way man, if they want to catch tax cheats then they will just have to find another way.