It's not really. Rangers have done a fair bit, & the fan base as well, in trying to eradicate songs & add ons. There's plenty songs now that are just Rangers songs, no reference to Pope's, Catholics or even Celtic in general. However, because of the uproar of the end of season 'friendly' that was arranged added to several poor moves by the board, certain Rangers fans have decided to take the opportunity to protest, both in terms of disrupting games and bringing out the old song book.
I appreciate today it was the full crowd the sang TBB, so perhaps the majority has seen it as an excuse to sing what they want. But don't pretend it doesn't happen with Celtic as well.