I agree with the OP completely. I myself don't like to commit two months in advance, but I would absolutely understand if someone wanted a firm yes or no by a certain date. You did say you'd take their "we don't know what we're doing yet" to be a no.
As an aside, after being on e-hell for a while I am making a concerted effort to commit to yes or no, not "maybe" or "I'll see" or if I am really uncertain, I check to see when they want to know by. Because it's true, "I don't know what I'm doing yet" DOES generally mean, "I want to see what my options are and decide closer to the time." Unless, of course, it means, "I might be visiting relatives on the other side of the country for two weeks over the holidays, but it's not finalized yet and if I'm in town I'd love to come!" But then if being out of town was a possibility, I'd SAY that, and still ask when the potential hosts needed to know by.