Because the stewards are letting them race, they won't be influencing what will be a classic season too much.
This is absolutely ridiculous logic though. Not applying the rules is just as much influencing the competition as applying them. What Vettel did
was dangerous. The other cars that have been pushed off at that corner have had their races ended. It seems, right now, pretty unlikely that Hamilton will win the race. The fact he was held up for several extra laps, and the fact that he had a collision, and the fact that Vettel didn't get the penalty he surely should have got, has shifted this race massively in Vettel's favour. It also sets a ridiculous precedent because a driver has now been allowed to get away with literally hitting someone off the track when they tried to overtake. What's to stop Hamilton from doing the same thing when Vettel tries to overtake him, punting him off into the rumble strips and destroying his race?
Is that really what we want? It may benefit a race here or there, it may benefit a driver here or there, but in the long term it destroys what makes sport fair. I mean, if they want to change the rules to allow that kind of thing, then fine. What you can't do is have rules that are explicit in disallowing X but allowing it anyway dependent on all sorts of mysterious criteria.