Fortunately I have never encountered that. It's kind of ridiculous because you could just as easily put a personal router behind the one 'provided' by your ISP. I do think the idea of a right of use is a good one, very good going on these guys!
The issue with that is that you end up with double NAT unless you happen to have the ability to turn off NAT on the ISP router (from my experience this is uncommon.)
Its actually worse then that. If they force you to use insecure equipment and to put your router behind theirs, their router can be hacked providing remote access to your internet connection without you knowing about it. And when you consider that many botnets are just after your internet connection...