There is a reason: simplicity. Either you do all the key management yourself, which in practice means 98% of the people won’t do it at all, or you implement a solution like they did and increase the risk of a small % (see my other comment) but you cover every customer.
Introduces some risks in terms of security. Privacy concerns are extremely minimal, because in any case you don’t control the setup of your other interlocutor(s).
Considering that the realistic alternative is not using anything at all and the fact that you have both options with Proton, it’s a win-win scenario.
One of the biggest risks is when someone knows your password. Your PGP encrypted emails that you want noone to see will be available to the attacker. Whereas if no such thing happened, the attacker wouldn’t be able to decrypt the PGP encrypted emails even if the attacker gained access to your account. Manually encrypting your stuff is better than having some random on the internet do it for you. It’s really just a tradeoff. Convenience or security? It’s not even hard to manually encrypt emails.