When Ronaldus Magnus did it, no one howled for his head

All presidents, indeed, pretty much all governors and probably mayors and even a few dog catchers do this kind of thing. The word "horse-trading" in politics is not meant to be understood literally after all. (Media Matters, for instance, has pointed out that, in almost a perfectly analogous situation, Ronald Reagan offered California Sen. S.I. Hayakawa an administration position if he agreed to withdraw from a Senate primary, just like Sestak. And like Sestak, he said "no thanks.")