If you want to browse the web with JS enabled, you'll be making lots of requests to Google and Facebook whether you want to or not (analytics, like/share buttons, etc). The most convenient way to stop them is to run something like a Pi-Hole so that those requests never reach Google/Facebook. An ISP offering that as a service sounds like a nice feature.
I would think the people knowledgable enough to know their browser is making requests to FB without actually visiting FB would also be knowledgable enough to block it themselves (with their hosts file, ublock, etc) and not need the ISP to do so.
I could be completely wrong, I’m no expert on North Idaho corporate politics; however, my thoughts are that the confusion is arising from your assumption that this is a practical move, rather than a political one
The users in question likely wanted the site blocked for everyone, not just themselves.
The email says users didn't want it in their internet "feed," so... this is a load of nonsense. I'm highly skeptical that anyone contacted them about blocking facebook in the first place.
Next thing you'd say "if you don't like somebody's tweets, can't you just not read their twitter feed?"! You're treading on dangerous grounds here, comrade.
That requires you to learn about computers and only technocrats like bezos and Tim cook know that stuff.
It's like people have never heard the word "boycott"