I think there's some middle ground, some grey area where whether it's alright is murky. It's kind of pulling the rug out from under people when the policy of a 3rd party provider abruptly changes and suddenly tons of messages become available to the company.

There are a number of things I might mention to a coworker over a private IM which wouldn't necessarily put my employment at risk, but would be awkward for management to suddenly have access to.

A couple made up examples:

"I'm super sick, but $boss is really pushing me to get the report out. I just want to go home and be sick all alone."

"I hate management's decision to reduce vacation days. No wonder we can't keep people around here."

"Did you see Tom's email? It's kinda awkward that he thinks he's a strong contributor to the group..."