Then why do you need open standards? Every major proprietary IM protocol has been re-implemented by open source software, including Hangouts: https://bitbucket.org/EionRobb/purple-hangouts
It might also be handled in the courts if the law includes language that specifically instructs to attempt simple, readable protocols to "facilitate interoperability.
XMPP, the thing Google is being roasted for dropping support for, is often decried as being very complex and hard to implement correctly. If genuinely open protocols often end up like this, how can you possibly punish companies for writing complex, hard to implement protocols?
"This is to prevent or reduce lockin, walled gardens, etc."
What I said. I'll add that it's hard to copyright API's to sue over compatibility when the API's and their implementation are released as open-source software.
"If genuinely open protocols often end up like this"
What are you talking about? There's all kinds of results from protocol design. You chose one of the worst ones. I was a PSYC fan over that if I had to choose an IRC replacement or open chat standard. A test for new ones might be trying to run it through LANGSEC's Nail, Cap n Proto or ZeroMQ to see if they handle it easily. Anyway, it's easier to do compatible implementations if you have source and can't be sued/imprisoned for imitating it.