> Yes, it's very much the expectation that I need to somehow be immediately available that makes it rude.
I pretty much agree with the other comments against people just saying 'hi' and nothing else. But this made it click to me.
I think many of the people who do this are the same who drop by your desk and ask for a random thing, without taking the time to ask whether you're available or not. This is the same thing, just moved online.
Also, bonus points for people who just say 'hi', you say hi back, and they never reply anything. Reminds me of people in person who would come up to your desk, say hi, while at the same time frantically typing on their phones and making you wait.