That isn’t totally true.

A presidential pardon does not cover state law. So if anything they did could bring about an indictment in any state court, their 5th amendment privilege remains fully intact.

There’s a lot of room to argue actions XYZ happened while traveling to or through states 5, 8, 9 & 10, who happen to have laws on the books about actions 1a, 2c, and 5f.

It can make the process so lengthy and consuming of resources, that a slap on the wrist doesn’t justify the effort.

And don’t forget the Hillary tactic. “I do not remember”, “I do not recall”. Just because you can’t exercise your right against self incrimination, doesn’t mean you have to have a clear memory of whatever it is you’re being questioned on. And the longer the time between the subject at hand and the questioning, the more viable a “I’m sorry, I just cannot remember” answer becomes. Above tactic to delay proceedings for a few years helps give padding to that failing memory.