Yes, one is a constitutional requirement, one isn't. But the point is a moral one, not a legal one. The president did eventually respond to the utterly ridiculous requests to prove his citizenship even though he didn't need to, but he did so because the issue became a major distraction. The state of Hawaii had already vouched for him, he was vetted thoroughly, but that wasn't good enough for the birthers, so the president said, fine, okay, here's proof.
But unlike his own father George Romney, who disclosed 12 years of his tax returns in his failed 1968 White House bid, Willard Romney will not disclose his tax returns and seems to be hiding something. If he isn't, then what's the problem? And what could he be hiding? Brad Friedman thinks it might be voter fraud.
Speaking of "show me your papers" Mitt? How about that SB 1070 law you like so much!
UPDATE with a "seriously, Lindsey Graham?" added: Romney Surrogate: ‘It’s Really American To Avoid Paying Taxes’
Just for fun, let's compare and contrast Romney to everyone else. Source: TaxHistory.org:
Since the early 1970s, however, most presidents have chosen to release their returns publicly. In the hope of making this information more widely available, the Tax History Project at Tax Analysts has compiled an archive of presidential tax returns.