Because they say so, sort of:
Sir Iqbal Sacranie of the Muslim council of Britain said Blair’s plan to ban the group Hizb-ut-Tahrir was not the correct solution. He claimed the ban would drive the group underground and would prove counterproductive.
Imran Waheed of the Hizb-ut-Tahrir insisted the group was non-violent and warned the government that if the ban went ahead there would be “serious repercussions”.
Etymologically, "repercussion" would seem to be an extremely unfortunate word choice, given the UK's recent "serious" experiences. I wonder which country Mr. Waheed would prefer to be deported to? And Sir Iqbal is apparently still reading from the old playbook which claims passive submission is the best response to a threat. I think his friend Tony Blair no longer agrees.