They do. They work with all of them.

http://www.businessinsider.com/the-story-of-joseph-nacchio-a...

AT&T just happens to have the oldest relationships and the largest infrastructure.

Which sane people call Man in the Middle and should not be allowed at all. I have seen people doing this Flexiable SSL with Credit Card data and other PII believing it is "secure"

Cloudflare may have started out with security in mind but their new services centered around centralization of key services (dns) and this kind of security breaking product means IMO they are a net negative in the world of Information Security

It's an easy box to check to pretend to offer HTTPS so you don't get penalized by Google. Before Let's Encrypt there was no free way to get a legit cert for your cat blog. Faking it via Flexible SSL was the next best thing.

Furhter before Lets Encrypt you would not have gotten dinged by google, Google only went that path when wide spread DV Certs where freely avaliable

Further still, the minor costs per year to get a paid DV cert should be factor when choosing to host your own content versus paying a 3rd party to do it, many of those 3rd parties provided SSL as part of their services.

There is zero need for a Man in the Middle for SSL,

Granted, like FB issues and others, we in this community need to realize that most users simply don't care (even many b2b ones) and not get upset when our users don't move their dollars on principle. For us it's a big deal, for many there are real, harmful issues going on in the world and volume collection of data by companies and governments is not one of them. And we can't make it so despite the deluge of articles by a supportive mass media.

EDIT: To clarify, you mentioned "other ISPs" but I want to be clear I'm talking about private pipes and not residential internet though I know they are often shared.