Because no one else gave you an actual explanation I will. The highest law in the US legal system is the Constitution. In it the president's official duties are described. Congress could not pass a law blocking him from doing his official duties as Constitution>Enacted Bill. To override the Constitution they would need to pass an amendment. Because of this any law enacted that may be otherwise lawful is unlawful as applied to the president if they were doing the act as part of their official duties.

If Congress could pass a law saying no one can issue pardons and arrest the president for doing so they'd have effectively stripped text out of the constitution.

As for protecting against treason and bribery, those don't sound like official acts. But they did cite an earlier case about Nixon that had previously set restrictions on how prosecutors may obtain information, that may benefit in any trial.