What Google did was in-house, behind closed doors, develop a solution, embed it in Chrome, push it out in production, and start using it according to their own specs right away.
Then they told other browser makers "Hey. Here's a neat ActiveX-like idea, which kinda makes the web platform-specific again, which you will have no say in how is implemented, because it's already in production, and unless you implement it as we see fit (and will continue to in the future), exactly as fits our browser-model and code (although it may not fit yours). Take it as is or we will be discriminating your browser on our web-services". And so they did.
It may not be proprietary by definition, but it's not "open" by a mile either.
Counter that with what Mozilla did: They proposed a way to make highly optimizable code-sections even faster, machine-code fast, in a backwards-compatible, web-friendly and portable way and invited people to join in. Those who didn't, would not suffer a lock-out, but those who joined could benefit from the work already done.
I don't think there's any point even pretending that these two actors are playing on the same moral level here. Google is acting scumbaggy and everyone but apologists knows it.
It's also worth pointing out (being maybe just a bit too pedantic) that asm.js isn't a standard. It's a subset of ECMAScript, which is a standard, and Mozilla has published a spec for it. The spec is more how to use asm.js, not how to implement asm.js optimizations. I'm not saying this is bad, but if you're arguing that Google's stance is, "hey here's the code for PNaCl and how to use it, good luck!", you can't argue that Mozilla's stance is any different.