I think they've handled this poorly, but at the same time I can see why they might have done it.
For people who had BT/TNT included in a legacy package, it was a package channel rather than an add-on.
As frustrating as it is, they do allow themselves wriggle room to add/remove channels from packages with 30 days notice, so what was 'advertised' or what they signed up for bears little relevance.
I suspect they've done this as a mechanism to quickly reduce TNT subscription numbers, as a point proving exercise and leverage in their negotiations with TNT. Something along the lines:
"...you want to increase the charge to VM per subscriber? Fine we'll cull the subscriber numbers so you make less money overall anyway..."
Of course at the risk of annoying people who've suddenly lost it, but I suppose they've decided the benefit is likely to outweigh the negative. Lots of Maxit customers probably never watched it, so more effective to remove in bulk and reinstate on a case by base basis.