Much though it might stick in the craw to say this, they don't have to justify it. All they do have to is not interfere in the police investigations. They're not bound by precedents, Tory Party rules aren't the same things as laws.
Boris has no doubt been fully briefed on this and the decision will be made to do whatever they might think is best for the Tory Party. Which will probably be to do nothing but leave it to plod and see what happens. I doubt they are trying to protect him why should they? It won't alter their huge majority, probably won't even cost them the seat though it will probably have a different bum on it eventually.