Why allow killers and survivors to play together

I've been part of 3 matches in the last 2 days where it's blatantly obvious one of the survivors was working with the killer.

In one match there was a survivor that would immediately start vaulting through windows, jump in and out of lockers, or drop pallets anytime they got near another survivor.

I'm another match one of the survivors just followed the killer around and teabagged every other survivor anytime they were taken down and hung

In the third the killer simply wouldn't chase or do anything to one of the survivors.

Allowing a single survivor and killer to invite and play with each other should not ever be allowed. At the very least A killer is highly likely to be preferential to the person they're playing with and more often than not it results in a killer and survivor pairing that is intentionally trolling the other 3 survivors. I would even say the only way a killer should be allowed to play with survivors is if it's all 5 participants at once. At least that way all the players were aware and agreed to it prior to the match starting.