Unions are different to other private bodies. They represent a university - places where the exchange of ideas should be most free and unencumbered by censorship.
Universities are the melting pots of ideas and the moulders of young minds. Limiting the exposure that these young people have to ideas that challenge them coerces them into following the university line.
University unions therefore, have a special duty to represent all views, and don’t have the privilege of choosing their agendas.