What people legally do with their own money is nobody else’s business.
Romney and Obama are not ‘people’ ,they are POTUS candidates in an election so this is fair (or at least expected) game.
But have you noticed that Obama campaign is constantly making personal character attacks on Romney and neither Romney or the Rove type independent groups are returning fire? In fact the Obama campaign keeps demanding Romneys tax returns, but no national Republican has the guts to reply demanding Obama for documents to his past.