PBA:
My understanding is that the buses are replacing other buses that would otherwise not be Gender specific. There were non Mehadrin bus lines (and still are) that go to these city pairs. If the mehadrin bus is the next one leaving, why should anyone be forced to wait?
BB 100a: ?? ??? ??? ?? ?? ??? ??? ?????? ??? ????? ??? ?????? ???
If only you were right and Charaidim and non Charaidim had no interaction whatsoever, both sides would be happier.