I'm not sure what you mean -- it makes sense to me that if you are paying for an email service, they would continue to charge you as long as you store and access those emails in their server, and they would have to take steps to prevent abuse from people who might try to store too much data. Can you be more specific about what the behavior is? Maybe you could show a good way that another email provider has solved this, and provide a helpful guide as to how they could implement that?

Can you name another corporate email provider that doesn't free up seats when users are deactivated? To my knowledge this is how all of Proton Mail's competitors charge for seats - at least all the ones I know of.

I haven't used many "corporate email providers", whatever that means, but I've used my fair share of "classic" email providers (standard mail stack + basic webmail) and most of them charge per inbox. If an address is deactivated, but the inbox is still present, it counts towards your quota (depending on your package, this might mean you're paying for it). If you want to reclaim it, the inbox with all its contents must be deleted. Since that is a highly destructive and usually non-reversible action, a few do require you to go through their support.

I personally don't like it and surely there's better ways of doing it, but it is definitely not unheard of.