Ideologically you are of course correct...but I would say Apple/iMessage is about as good/bad an actor in the IM-space as their competition.
Maybe they wanted to, but it was too onerous back then to force client software updates that they had to continue supporting reverse-engineered versions of the protocol?
MSN Messenger and Yahoo IM both cried foul about this and called out for interoperability, but then turned around and denied third-party clients access to their own networks once they got big enough.