I thought it was just common sense. I used to live in a rural area with buses every hour, didn't drive. And there was a time where I couldn't get on the bus cos it was too full and I didn't have a foldable pram at the time. I wouldn't stay on if a wheelchair user needed it. After a while I got a really easy folding pushchair for this reason so I didn't have to worry about it. Once I was getting in bus and wheelchair user was as well, there was room for us both in the anyway but I knew he was worried cos he said something so I told him it's ok I can fold it up
I do think bus drivers assume you won't fold it up though. If there's no pushchair space they always say there's no room I have to say to them it's fine I can fold it. Presumably because most people would rather wait for the next bus than fold it up