The American people have a right to be as fully educated as possible when it comes to choosing their next President. And a candidate's tax returns being public is not only a tradition of honesty, respect, & a sign of openness to the people, but also reveals the undeniable reality of their own economics:
-Does the candidate pay taxes?
-If so how much?
-Does the candidate donate to charity?
-Does the candidate have any vested business interests in foreign countries?
-Has the candidate taken out any loans from foreign banks?
-Has the candidate ever declared bankruptcy?
-Has the candidate taken advantage off an excess of tax loopholes in order to pay as little or nothing at at all
Therefore any candidate running for, or sitting POTUS should be, by law required to release their tax info.