What makes you so sure about that though? Who explicitly told Dumb Donald he can't do that, or anything else? Certainly not any of these cowardly Republicans that take turns pretending to take a stand against The Great Pumpkin's latest deplorable act before they're back fully supporting him and doing his bidding a week later...
I don't expect any of them to do the right thing when it comes down to it
His legal team, the motivation would be keeping him in office and shielding him from criminal charges as much as possible. Trump is erratic but make no mistake he is not trying to go to prison and ultimately I don't believe he will. I think he will be forcibly removed/resigns from office, that is inevitable. The game now is to delay that in order to shield and shift as much liability as possible.
Trump f*cked himself firing Comey in a major way. At that time, the FBI had to put together a strong collusion case and the money laundering investigation would take years. He'd be long out of office by the time they would be ready to bring charges against him but he gave them the golden ticket by firing Comey; he gave them an obstruction of justice case. Being easier to put together than everything else and also could be used to justify and expedite the evidence gathering of multiple other crimes.
I am certain that piece of the puzzle has been drawn out for him that firing certain individuals is going to have consequences similar to the Comey firing. Which explains why he didn't accept Sessions' resignation after demanding it as previously reported.