it's just people. people who say hello to you when they pass you on a bike are just as likely to be the ones who smile and apologise when they bump into you on the tube.
some people are devoid of social skills or just plain miserable though, sticking them on a bike doesn't change their crappy personality.
I was coming back from an evening ride the other day and stopped in a little village for a pint. I had a full face helmet with me and propped a 7" FR bike next to a load of shaved roadies.
my friend and I had the picnic table next to the roadie lads. Their ages ranged from around 30-50 and they all looked pretty fit, and their gear was high end.
We didn't get the usual smirks and rolling of eyes from these guys though. Two of them were up like a shot and asking loads of questions about both bikes. Neither of them claimed to be an MTBer, but both were pretty knowledgeable about our kit. All (save for one miserable git) were really friendly and seemed genuinely interested about where we had been and what we had been riding. We were equally impressed by their 70 mile loop as they were by the mobile phone videos of us sessioning a road gap.
Some might suggest that we're all cyclists and that's why we got on, but at the end of the day, these guys were just decent chaps. It could quite have easily been 6 **** sitting next to us.
It's quite rare that I don't speak to another MTBer if I bump into them on the trails.