The Proton VPN feature seems to require a Proton Account.
Blocking the Sync on anonymous email probably doesn’t make much sense itself but a Vivaldi account can also link to the social[dot]vivaldi[dot]net Mastadon instance and [@]vivaldi[dot]net email accounts.
airmail[dot]cc seems to show up because of randsomware attacks in brace search before the actual email service so it makes sense that they’d block it on any service other than sync. They might simply not have developed a way to do that without blocking sync.