Let people buy whatever they want--it's their money, their choice. If you agree that the quality of sound is subjective, then there's absolutely no argument and no reason for a thread title proclaiming ("proving") that the average consumer is "deaf".
For example, from reading various threads around here, I'm certain there are quite a few respected members who would take the Beats over any Grado, any day. Are they deaf? Does that frustrate you? Sure, the sound of the Beats is colored, but what heaphone isn't? And what about Grado? Why is somebody's decision to purchase one of the most colored headphones simply because s/he enjoys its sound so much more respectable than someone who purchases the Beats for the same reason? A basshead who purchases a DT 770 Pro for its bass would definitely not get as much crap from this forum as someone who purchases the Beats for the same reason. Seriously, the attitude that some people have around here has got to stop.
One thing you should not do is look down your nose at these people. The best thing would be to inform them, but ultimately understand that the choice is theirs. Getting angry and frustrated over trivial things like people's purchasing decisions only makes you and the people involved in this hobby look bad.
By the way, I don't mean to single you out, as there have been plenty of threads about this same topic (noted above).