What would be the obvious reasons? (I'm not being flippant here -- I'm genuinely interested in what arguments people have to not allow servers on that network)
From a business perspective you'd want to charge extra. Just because you can, but also because you want to discourage excess bandwidth use. The internet APs the carriers sell get deprioritized relative to phones when necessary and the fine print generally forbids hosting any services (in noticeably stronger language than the wired ISPs I've had).
Isn't that already the case with limited plans?
For example, mine has 40 GBs and I'm pretty sure it counts both upload and download, because I generally consume very little, except for one week when I was on holiday with no other internet access and wanted to upload my pictures to my home server and didn't otherwise use the phone more than usual.
Sounds farfetched? https://www.theregister.com/2025/06/03/meta_pauses_android_t...
The main difference is there's usually limited airtime capacity for clients, especially highly mobile ones. A server could easily hog quite a bit of the airtime on the network serving traffic to people not even in the area, squeezing out the usefulness of the network for all the other highly mobile people in the area. This person moves around, pretty much doing the equivalent of swinging a wrecking ball to the network performance everywhere they go.
When its being sold as a fixed endpoint though, capacity plans can be more targeted to properly support this kind of client. They're staying put, so its easier to target that particular spot for more capacity.
Similar to many industries, their business model is selling monthly usage, while simultaneously restricting the actual usage. They are not in the business of being an ISP for people running software on their phones.