There are two possible reasons that stand out a mile for why those newsgroups would be removed, one is the severe impact that they have on the servers and bandwidth which affects all other newsgroups and the other is nicely pointed out be a member name in this thread. The ammount of illegal software/CD images being made available through binary groups.
NTL may just be having to re-work the newsgroup servers again to balance the workload or it might be that they are trying to purge some of the warez off their system
