Well, it's his prerogative on who works at the WH....maybe she wanted to go, who knows.
Yep. In the oil patch as consultants we sign something that says that even a contract can be terminated at any time by either party, with or without reason. Works for me, as a rule.
If she is in a position to gather information that can be used to thwart the administration's agenda or otherwise impede the operation of the Office of the President, and is loyal to other partisan factions, it may well be prudent to let her go and replace her with someone whose loyalties are less questionable. This administration has been plagued by leaks early on, and it's time to seal the cracks, or any which might appear.
A generous severance package and statement that is virtually a recommendation is not a bad way to go. There will be those who try to make an issue of this (sexism/racism), but I'm not buying.