Yep, that's about it! A bit like drivers who don't acknowledge when you've given way to them. Rudeness.
I guess it could be that they're approaching a corner or planning another move which demands total concentration. You wouldn't want to overshoot a corner just for the sake of politeness! But, at the end of the day, some are just miserable! If you're riding in a group, it's different - etiquette is for the oncoming rider to acknowledge the first and last bike, or he'll have a sore neck and look like a half-wit!
Keep waving or nodding. I prefer the continental approach - a small gesture with the left hand - but it wouldn't work too well for us in the UK.