But I should have been more clear. not all preinstalled apps are like this, just a lot of the less important ones (at least on google-branded phones, I have no idea what samsung or lg are doing to facebook). I believe it does require the developer of the app to actually implement it though.
But either way there is literally no benefit to removing those apps from the system partition. You won't get the space back (different partition), and if they allow users to delete these things they can very easily get their phone into a state that you can't recover from (factory reset won't work because there's nothing to reset to).
I'm 100% with you that non-essential apps should be uninstallable as much as possible, and preferably I'd like them to not be installed at all, but that's an entirely different issue than what Google was just fined for.
At the end of the day, i MUCH prefer Android's handling of removing/disabling factory installed apps over something like MacOS where if you "factory reset" the device, you can't use it again until you connect to a network and re-download the entire image.