This wording annoys me. The websites have nothing to do with it. Google choosing to turn it's browser into spyware that leaks information about what you used to do with it isn't the websites fault, the webserver doesn't do anything and doesn't have anything done to it, there is nothing for it to opt out of.
Google chose to give websites a way to request that the users browser doesn't include the fact that they visited this website in it's cohort calculation. That's fine, but the messaging around it is a transparent attempt at shifting the blame. It's not the website opting out or in, it's the website acting as an uninvolved third party bystander asking google to stop. Asking why a website didn't opt out is equivalent to a thief asking "well why didn't you stop me?" to the person looking on from the sidewalk.
We shouldn't accept this messaging. We should be very clear that Chrome is the entity spying on you, not the website, and that the website has no power to decide whether or not chrome spies on you, only the ability to make a polite request that it doesn't (or more accurately, does so less).
FLoC is only opt in for testing the proposal[0]. As a sibling comment says this is technically performative but publicly signals a stance against the proposal.
Though we also shouldn't forget that Amazon loves third party tracking and happily falls back to IP address associations if cookies aren't available.
Edit:
[0] https://developer.chrome.com/blog/floc/#take-part-in-a-floc-...
You can opt-in to actively be a part of FLoC, but if you don't opt-out, Google may randomly choose you to be part of their testing.
Edit: I think your point may have been from the perspective of a website owner. Sorry.
> shoot the messenger, it won't fix Firefox bugs or help their market share.
Your message doesn't really contribute to it either. If you hate Firefox with a passion and wish that Alphabet dominates the world and turns us all into clicking zombies, keep spreading it. Otherwise, better not.
Firefox is doing the damage to themselves.