Elizabeth Warren and Scott Brown tax returns released, now where are Mitt Romney's?

Now, about those tax returns, Mr. Romney ...
The candidates for Senate from Massachusetts, Democratic challenger Elizabeth Warren and incumbent Scott Brown, both released several years of tax returns today to general yawns. Really, there's pretty much nothing there.

Now that we're done with that bit of campaign theater, let's talk about the interesting part: why Eric Fehrnstrom, who advises both the Brown and Mitt Romney campaigns, thinks personal tax transparency is so important in the Massachusetts Senate race, but doesn't matter in the presidential race.

Fehrnstrom isn't the only one. Here's Karl Rove in a Wall Street Journal column, on vetting a vice presidential candidate:

Having played a role in this process, I know that if done well this will be a political proctology exam for each individual considered. Ms. Myers and an army of lawyers, researchers and accountants will examine the person's every public statement, vote or executive decision; they will review tax returns and financial records; and they will scrutinize friends and associates. They also will ask finalists what in their background could embarrass Mr. Romney if it came out, because it will. [...]

 But such political decisions run into one hard reality: Running mates haven't decided an election in more than a half-century.

So, even though the VP nominee doesn't decide elections, all the candidates will undergo "political proctology." But the top of the ticket gets away with refusing to bend over?