There are existing mirrors for Fedora and Ubuntu packages in China, which are used because mirrors in other countries are often blocked. I’m sure there are no legality issues—the issue in the case of flatpak and china in particular is that China blocks Fastly because Fastly does not host any POPs in China. This is why Cloudflare, for example, has their own network in China that international users can pay to use. There’s no legal issues here, just logistical. Besides, as previously shown, people do (with great difficulty) managed to bring up their own flatpak mirror without any consequences for a few years now.

Besides, there shouldn’t be legality issues for businesses wanting to host their own mirrors for compliance issues.