Why is protonmail domain disallowed for signup

I found that I’m not able to sign up for protonvpn using protonmail account: I get ‘email domain not allowed’ error. Why is that disallowed?

EDIT: OK, so it seems like I can login with my existing account. How am I supposed to know that both protomail and protonvpn use the same account? Since the error message goes to the trouble of disallowing protonmail accounts, won’t it be more helpful to indicate that ‘you can sign in already with that account’?

You can just sign in. You don’t need another account

Because you should just be signing in with your Proton username.

Because you’re trying to create a second account using your first account with Proton. Your proton email already works for all Proton services including VPN.

Do you need another account for a special reason?

Because by having a ProtonMail it means that you actually already have a Proton account and thus you can log into ProtonVPN with that.

Back when I signed up, Proton recommended using separate passwords for each of their products. Wouldn’t that require separate accounts?

because companies cant data mine protonmail they dont like it and it is used by people that may or may not break the law so it gets a bad rep

That mainly applies to Proton Pass, to not have the Proton account password inside your Proton account… Don’t think it applies to VPN.

Proton Pass didn’t exist at that time.