FTTH is somewhat open in Canada. You don't have to use their router, but you do have to use their SFP. (Why? I'm not entirely sure. AFAIK it doesn't send anything back to them)
The security is mostly provided by the fact that most internet is cheap enough that the mere price of acquiring an SFP or GPON ONT + the time it takes to reverse-engineer and hack isn't worth it. Also, breaking into unterminated fibre itself requires a fusion splicer that will set you back a couple grand, which is enough to pay for 1+ years of internet in most places.
Same with DOCSIS, the security is mostly obscurity/annoyance but anyone dedicated will easily bypass it.
It's actually quite a good setup, as it allows power users to swap the router with anything SFP-capable and provides a standard interface across the demarcation line.