The reason people are incensed about FAANGMP doing so is because, in their respective markets, they're monopolies.
No one would care if Google banning a developer meant they could list their app through a non-Play app store with decent exposure, or a non-App Store at all.
But that's not the reality we live in.
So it's more like if Walmart moved into my podunk town, put all the local shops out of business, and then banned me.
There is speculation that Google will ban sideloading in the near future, too. That is, it will extend its Advanced Protection model to mass-market Android. Then, sideloading will only be possible for that tiny minority of nerds like us who know how to use ADB and install an .apk over the command line.
No one would care if Google banning a developer meant they could list their app through a non-Play app store with decent exposure, or a non-App Store at all.
But that's not the reality we live in.
So it's more like if Walmart moved into my podunk town, put all the local shops out of business, and then banned me.