Really annoying that you’ve been downvoted for a very reasonable take.

and the buses are at their busiest so discouraging extra users at that time is beneficial.

This is the key justification. However, the issue it points to is that the government are putting the desires of a business (primarily First Group) before the needs of the population.

The goverment should make law for the benefit of the people, and businesses should work within the law in service to the people, especially people that pay them. The bus companies should run more busses at peak times to meet the added demand disabled people bring.

Bus companies get massive subsidies and the businesses are stuctured in such a way as to squirrel their profits away so they can demand more. That the government, the people supposed to represent us, are capitulating to them further is an insult at best.