Because as it stands right now, AT&T sells you access to their network. What happens on their network is for AT&T to decide. With the FCC striking down net neutrality [1], AT&T is probably testing out the waters.
[1] According to google, it's defined as:
"the principle that Internet service providers should enable access to all content and applications regardless of the source, and without favoring or blocking particular products or websites."
They've done this before, during, and after net neutrality. AT&T regularly blocks entire ranges at the IP level because they are "suspected of cyberattacks." I've frequently had issues with web hosts who are blocked only on AT&T, and this was the case in October-November (before the FCC vote) while I was launching a new site.
If you can construe some horizontal where Cloudflare and AT&T are competitors it could of course still be illegal for AT&T to block the others services simply under antitrust law.
NN hasn't actually been struck off yet though, it's still on the books isn't it? Pai needs to sign something and for some reason he hasn't. It was in the news a few days back.
