Well, you're right, but let's put this into context: the only viable alternative at the moment is Microsoft's ActiveSync, another proprietary protocol[1]. IMAP IDLE apparently isn't sufficient for power conscious mobile devices. So what alternative do they have? FastMail's already pushing their own JMAP[2] protocol which I imagine they'd be very happy to become a standard. Perhaps everyone should wait for a standards body to come up with a solution, but personally I'd rather they be pragmatic and actually help end-users until that happens, even though I agree with you on principle.
[1] ActiveSync does have open implementations, so it's not like you're locked out of push notifications to iOS Mail if you are a private individual or small business who wants to use this functionality, it's just that for whatever reason Fastmail doesn't want to use ActiveSync, hence this alternative. But as an iOS end user, both solutions offer the same experience.
[1] ActiveSync does have open implementations, so it's not like you're locked out of push notifications to iOS Mail if you are a private individual or small business who wants to use this functionality, it's just that for whatever reason Fastmail doesn't want to use ActiveSync, hence this alternative. But as an iOS end user, both solutions offer the same experience.
[2] http://jmap.io