It's an important piece of pedantry though, because it shapes the debate. Label something a tax (and in this case entirely wrongly) and it sounds like the government is taking away money that people are rightly entitled to -- which, in turn, has stirred up all the hatred and protest against it.
Giving it its proper name at least allows the debate to be conducted in the right context; that this is a reduction in benefit for those people who are occupying a house with more bedrooms than there are occupants -- and, hence, people whoa re living in houses that are too big for them, really.
I'm not saying I like the idea of cutting benefits for such a reason (it can get horribly arbitrary, for example) -- but at the very least it should be recognised as not a tax. Calling it such is only for propagandist reasons.