> There is a grey area here of calls from businesses which the customer is signed up to

Pretty much exactly how it is in Switzerland. I once had a rather unpleasant experience with my old internet provider (Sunrise) where I asked the telemarketer who called me not to do so again, and was told they "had a right to do so, because I was a customer". I immediately cancelled, quoting this as the reason why (Sunrise would later try to tell me this never happened, but that's another story).

With that said, I think it does make a difference: if some random company calls, they can't lose my business because they don't have it; if a company I am a customer with calls and refuses to stop when I ask them to, they risk losing my business. I don't know if this is the reason why, but I barely get any telemarketing calls (never heard of anyone else here who has either).