Why does BHVR allow/tolerate stream sniping?

Because ultimately attempting to police it is futile. Stream sniping is something that is already incredibly difficult to prove and in addition an accusation that gets thrown about by lots of entitled streamers who think that just because they lost they have to have been stream sniped. Which means it's not going to work at all whichever way they decide to police it.

If you need considerable evidence to report stream sniping, good luck getting it. Snipers can easily hide their names/ use alt accounts/log out/have plausible deniability (eg: I was only stopping by before the match/I saw your scratch marks). Unless you literally get recorded evidence of your opponent sniping you it's a futile endeavour.

If you DON'T need considerable evidence to file a report, streamers will be able to get their opponents investigated on the suspicion of stream sniping, even when they have no evidence whatsoever that they did. It's just added paranoia to the already considerable paranoia and potentially bad attention that comes with versing DBD streamers. No need to add fuel to the fire.

It's a lose-lose situation, basically.

Because it's unenforceable and avoidable

use a delay and use anonymous mode.

For the simple reason it's impossible to actually prove in any way unless the person doing it admits to it.

I remember during the earlier PUBG/Fortnite days, there were controversies as the games actually tried to dish out punishments for it based on situations that merely seemed "suspicious" but didn't have any undeniable evidence and then it was later shown there was no streamsniping involved. It was just a case of the accusing player not knowing the full context, what the "streamsniper" actually saw and heard in-game and so on.

I have zero doubt that if there was some way to conclusively prove it 100%, more games would ban for it.

As others have stated, even if you decided to do it, how do you even prove it?

I've had friggin hunches make me check a random locker in a match only to find a survivor hiding. There was no in game reason to make me do that. I didn't see marks or blood or hear anything. Just a gut feeling that something felt off. That's not even taking into consideration just experience and knowing weird places people like to hide that I randomly check from playing against others.

To someone streaming, it very easily could look like I sniped them because they didn't see any good reason why I acted the way I did. If you stream, this is a risk you take on.