[pavon]

To a large extent this immunity is inherent in the enumerated powers. Consider this general case: The Constitution enumerates specific powers to the executive branch, and hence President. Congress passes a law that makes those same actions illegal if performed by a normal citizen. If this law applied to the President, then that would mean that Congress could nullify the enumerated powers granted to the Executive, making those enumerated powers meaningless. So it makes perfect constitutional sense that actions performed by the President as part of his job that that are within his enumerated powers cannot be made illegal by Congress.

That core concept of immunity is pretty solid and essential, it is the details that are problematic, in particular the fact that the courts have interpreted "official" acts so broadly in the past in cases of qualified immunity makes one worried they will do the same here.