Jonathan Chait in the New Republic effectively clarifies things:
So the accusation is some kind of quid pro quo in which Sestak would receive a job in return for quitting the Pennsylvania Democratic primary. This is ridiculous. You can't offer a Senator, or prospective Senator, a job in exchange for them abandoning the Senate, because accepting the job inherently means leaving the Senate. You can't be both a Senator and an executive branch employee. Last year, the White House offered a cabinet job to Senator Judd Gregg. This was not "in exchange" for him leaving the Senate, because he had to leave the Senate to take the job. Moreover, Gregg briefly accepted the job in exchange for a promise that New Hampshire's Democratic governor would appoint his Republican chief of staff, not a Democrat, to replace him. But nobody suggested that this deal was illegal or unethical.Thank you.