It's a question of the topic. Some topics need to be handled synchronously. And if you don't answer in a conversation this is legitimately perceived as "rude".
If the topic of this conversation is whether or not IM should be considered synchronous, then this argument doesn't hold up. If I take one side and suppose that IM should not be synchronous, then obviously the mistake in your case is on the individual that tried to use it for a topic that needed to be synchronous. The not-responding person is not wrong in that case since they're ostensibly using the method of communication as intended.
Part of it is that, in many cases, we've normed not calling people on the phone out of the blue. And, while some may disagree, we often do need some reasonably real-time communication channel. Someone may not see a message instantly but it is reasonable to have some mechanism in a business setting to generally reach people quickly--if for example, there're supposed to be someplace.