Why was the pardon of Nixon controversial

When Gerald Ford issued Proclamation 4311 (the document in which gave Richard Nixon full and unconditional pardon), he was excused from any crimes he may commit against the United States while president. This was controversial because there was nothing stopping Nixon from committing treason against the United States. People believe the law should apply to everybody equally no matter what the standards nor what position/title you hold. The other side of the argument is that because he's the President of the United States and holds all this power, he's "untouchable" in a sense, and/or doesn't deserve to be held by the same law that every other citizen is under.