It's that presidents can't be prosecuted while in office and it's not a conservative legal theory. It's a pretty cut and dry 101 thing. Federal prosecutions are conducted by the executive branch. The president is irrevocably at the head of the executive branch. While lower authorities like the Attorney General can recuse themselves because everyone is ultimately under the authority of the president, the president can't recuse themselves. There's no one to recuse to. And you can't be at the head of your own prosecution. That's a fundamental fact that applies to everyone.

The real question is, why do people think it's such a great idea for a President to be prosecuted by their own DOJ? This was always so weird to me, when people would lay out the case for Trump to go to jail. He's corrupt, he's a liar, he abuses power...so let's have the people under his authority prosecute him?