That’s apples to oranges. If Trump pardons Flynn it would be comparable to Bush pardoning Weinberger, and there’s no question it would be within his Constitutional authority.

But impeding an ongoing investigation is different from using a pardon to absolve an individual from the legal penalties of a crime.

If we follow your reasoning, than Nixon and Clinton did not obstruct justice either, since the president cannot possibly commit obstruction of justice. I suppose one could make that argument, but there’s clear precedent saying otherwise.