The person in the pushchair can't walk, the person in the wheelchair can't walk. In my eyes they are the same. You may say that the Mum can push her buggy, but so the wheelchair can push their chair, so why should one have priority over the other?
Many have said the buggy should be folded to allow the wheelchair user on. Maybe you should try taking a small child shopping then take the bus home if you think it's that easy! You would have to declutter the buggy of bags first, then give your baby to someone else to hold, then fold the buggy and then stash it, get your baby back, and then hope there is actually a seat left to sit with them. I would not be happy to give my child to a perfect stranger to hold, assuming that there is even someone willing to help you!
In this scenario, if I had one stop to go and had to do the above , I can understand why mums refuse. As someone said before, the law states equality not preference.