In my experience that’s usually the case for XG-PON and XGS-PON networks. Because you’re sharing one port on the OLT with up to 63 neighbours. Though I think most build outs aim for 16 or 32 splits.

Anyway they don’t want to risk you sending when it’s not your turn or disturbing your neighbours connection in any other way, they make you use their ONU. Basically the same old story like with the coax cable modems. Just because some idiot had to go and turn fiber back into a shared medium to save some strands.