I agree, and that is a problem. The rules and regulations are different in different countries. In Austria for instance the ISP can force you to use a specific DOCSIS modem or ONT but they have to provide you with a transparent way to connect to it (bridge mode etc.). Which from where I'm standing is a good tradeoff because it gives the ISP the flexibility to do mass migrations without having to consider very old deployed infrastructure.
With PON I think it doesn't matter all _that_ much but for instance people running ancient DOCSIS modems and limited frequency availability has been a massive pain for people stuck with DOCSIS infrastructure that want more upstream and can't.