the reason is how do these companies get away with it?
Well, technically they shouldn't as you need to jump through hoops in the UK to be allocated premium rate numbers and its generally made quite clear at all stages what you can and cannot do.
But of course, scum ignore that ... and so much like email spam, its then a case of adopting a reactive policy since the proactive element has evidently failed.
Much like email spam too, I suspect if you look hard enough, you'll find companies willing to accept bulk sender customers and provide them with a premium rate response number whilst turning a blind eye to what they use them for.