You need to see the bigger picture here. Protonmail is funded by a "donation" and "subscription" service and would always lack the funding to build capacity like Google. When they run out of resources, they rely on "donations" to increase their capacity to allow for more people to join their service.
You have a good point there. This indeed explains that in some cases (so far I haven't experienced it yet) they don't allow free registrations due to the lack of funds to further expand their whole service.
That being said, I am sure that when you happen to register a paid plan, that you at all times can get their service to work, and thus can avoid waiting for free registrations to become available again.