As add-ons extend the computer version, they must be licensed under the AGPL3 or a compatible license. If you do not explicitly state a license, it will be assumed to be AGPL3.
Maybe this touches on the same topic that was raised on the reddit earlier, about the new AnkiHub plugin.
In that case I assume they’re in the clear because the add-on itself should be freely available but you require a subscription to use the collaborative deck features.
AFAIK, AGPL was intended to prevent people from modifying open source code, hosting it on a server, and failing to provide the modifications to others. That’s not what’s happening here - assuming the add-ons are being released under a compatible license, I do not believe they are breaking the license by making network connections to proprietary services.