That‘s the legal aspect of things. That is of course totally normal. Every ISP has to follow that. Blocking other sites at your own will, just like swisscom is doing here, is not.
Having users that simply do not wish to be blocked by your blocking service for „gambling”, or those that simply do not trust your DNS servers should still be free to use public DNS servers.
That still does not answer why you as an ISP try to convince your customers to not use Public DNS Servers, or „not seeing a reason“ in them doing so.