No, they do not allow users to enable Private Relay at all because Apple allows carriers to determine whether it's available or not. Even FaceTime over cellular is still something that carriers get to decide whether to allow or not, although I'm not aware of any carriers that don't.
From my limited testing, carriers are whitelisting traffic for high-bandwidth. When I establish a vpn tunnel on my Tmobile sim card, bandwidth drops dramatically. Presumably because they can't inspect it.
There are legitimate reasons why a specific business network might not allow it. For example, if you're on the employee network of a bank or hospital, it's very likely that your web connections are going through a proxy to make sure you're not sharing confidential data, and to block malware and such. Private Relay would go around those proxies. Allowing networks to opt out of Private Relay, then, is a better business decision than having enterprise networks just block all iPhones.
Corporate networks makes sense, but giving carriers the ability to disable it on the phone (i.e. not via blocking mask.icloud.com) doesn’t make sense. It’s not like personal hotspot where it allows you to bypass network policies, except for maybe the streaming shaping (but how long did they think that would work anyway?).
These deals are old. FaceTime when it came out was in the era of 3G. FaceTime over 3G could be a bandwidth hog.. and iPhones were not nearly as popular, so the negotiations were more give-and-take.
