Why are illegal curry shops tolerated?

I know this is not a problem specifically isolated for curry houses, but there definately is a high proportion of workers in curry houses, who are illegal immigrants, and are cycled around curry houses to act as cheap labour, often given bed and board, and a pittance to live off. But they have access to fake doents and driving licenses. One man in a resteraunt near me boasted that if he is banned from driving on one, he has seven more to use... how does this go un-noticed? Recently a curry house near a friend was found to all the kitchen workers in this country illegally... why is this blatant people-laundering system not more thoroughly policed?