They do not have to subscribe to it - it submits through the patchwork of local council crap apps and then you get the alerts from the council directly. That does mean that people have to reply with FixMyStreet's monthly "is it fixed yet?" questions to update the status, though.
Councils can of course deliberately ignore reports sent through it, but doing so does not protect highways departments from liability under the Highways Acts if damage is then caused as a result of a problem that they ignored the report of, so it's a rather risky strategy. I also suspect that any council ignoring a government-funded app like FixMyStreet is going to ignore CTC's Fill That Hole and other third-party apps.