Earlier quoted context omitted.
Popular OSS licenses were designed in an era to allow direct customers (businesses) to directly install and use the software, and redistribution means only distributing the package. Nobody at that time knew or predicted that SaaS (let alone cloud computing) would be a thing, that large, profitable middlemen will directly host the software, pass through just the API, with zero customer exposure to the running instance…
> The first version of the Affero General Public License (AGPLv1), was published by Affero, Inc. in March 2002 Stallman strikes again!
https://opensource.google/docs/using/agpl-policy/
It appears that they are concerned that it may be argued that if for example YouTube uses an AGPL DB that they would be required to release YouTube under AGPL.
I'm not saying that the interpretation is correct, but if companies can't be sure they won't use your software.
And to be honest IANAL but their interpretation seems reasonable to me. If you put Mastodon behind NGINX I think it is fairly easy to argue that the user is "interacting with it remotely through a computer network". So now if you have a theoretical AGPL Elasticsearch and your site sends the user search query to it and returns the user the results it doesn't seem clear that this is a legally meaningful difference. I certainly wouldn't want to argue that in court. I'd just use different software.