Do they really though? I have never seen any wheelchair user be told they can't get on a bus / train by anyone (with the exception of the social media clips.)Perhaps not. But if it is possible, then they should do so. Reasonable adjustments only need to be made for those who cannot do the default, they are not and should not be offered to all.
The problem is that people refuse to move or are rude about it. If there are no tip up seats the argument can't ensue. It makes disabled peoples' life easier and it makes the driver's life easier. It's only three seats, rarely will the bus be at exactly the level of capacity where they are necessary but where it wouldn't be better as standing space.
Maybe it would be best to have a wheelchair user who has actual experience to advise on how big of a problem this is, as everything that everyone here is saying is just anecdotal. I have a wheelchair bus user who I'm communicating with in relation to this issue so let me ask him.