In an alternate universe, AOL is shaking in its boots because they don't want to be forced to allow users to cancel their service...
In all seriousness, I can see some potential problems with this. You absolutely should be able to delete your account, but 1) some situations may put people in a position to not understand the implications (if I delete my Google account, I would then also by design be "deleting" any other account tied to it with SSO), and 2) any company can simply mark an account as "deleted", very much like how filesystems just mark files as deleted but the data persists. And of course, 3) if you're "signing up" for a private service, e.g. at your employer (even though it's using a publicly available app), you might not necessarily have permission to remove your account. (If you use Slack at work, your employer probably should be allowed to restrict you from deleting your Slack account in their workspace, even though you might sign up for it in the app.)
Another problem case would be apps that connect to self-hosted systems. If you self-host a cloud service like Nextcloud, and the app lets you sign up for new accounts, does the app also have to require the server admin to allow account deletion from the app?