No, it's reasonable in some cases. If the ISP supplies the router then they're perfectly within reason to put whatever restrictions they like on it. It's a bit like the way BT don't allow you to fiddle with the master socket.

(for non-English: BT are the phone company in the UK. They supply one phone socket, and wiring from that to their exchange. That wiring and socket are their responsibility, and woe betide anyone who messes with it. Anything else, like phones, extensions and microfilters are your responsibility, and they don't care what you do with them as long as you only use approved stuff and dont kill their system)