Yes, this is a real problem. I turned up at the cinema without any idea of this charge. I rejected to pay and walked out.
Since then I've researched the web. Older T&Cs from 2007 don't have the 3D exclusion:
"3.6 Subject to availability and the other provisions of the Conditions, a valid Cineworld Unlimited Card entitles the Subscriber to access selected Cineworld cinemas every day of the week (subject to the Cineworld cinema being open),
for all films, and all presentations (excluding Delux screens, private hire occasions and events not open to the general public) and entitles the Subscriber to one seat (excluding VIP seats where available) for any performance subject to
availability. The Subscriber acknowledges that all film showings are subject to normal restrictions in terms of cinema age certifications, and that Cineworld