You say "why should I have to pay", but they really haven't said or suggested anything about how they'd rather you paid for anything. They're talking about having an option to supply one's own device, not about requiring so.

The common rationale behind this I'm aware of is that an ONT device is technically a computer with persistence, hosting arbitrary code and data that you cannot (or at least not supposed to) audit or alter, despite being on your premises, operated on your cost (electricity, cooling, storage), and specifically deployed for your use. These properties hold for SFP modules too in general, not just SFP ONTs (they're all computers with persistence).

The catch is that this is further true for all of these kinds of modems.

The counter-catch is that despite that, for DSL specifically, you could absolutely bring your own modem, hw and sw both.

The counter-counter-catch is that with DSL, you were not connecting to a shared media, but point-to-point. This is unlike DOCSIS and GPON, where a misconfigured endpoint can disrupt service for other people, and possibly damage their or the provider's devices and lines.

That's all the lore I'm aware of at least.