As the cap wording is in the AUP in black and white it can be enforced at any time that ntl decide use is a problem.
ntl recently told anticap that the policy was a proactive step. They were anticipating future problems. The very very few customers contacted by ntl (none have admited to being contacted) no doubt fell fould of the much more traditonal don't do anything to affect other customers service.
ntl have also said they want customers to have an "enjoyable experience".

I want to know how I am supposed to have enjoyment when I am constantly looking over my shoulder wondering if ntl are going to make tell me off! ?