Eww yuck! They are rubbing their gland? That's really sick sounding.
I've only seen my cat do it not more than a dozen times in his lifetime. When I see him do it, I shoo him away and then I wipe down the area with disinfectant. I didn't want "cat ass" on my furniture. But now that I know it's a GLAND... well, I'm not sure, but I think that grosses me out even more!
Eww. I think it's the word "gland" that does it for me. Maybe his gland was removed during the neutering? Do you think? Maybe he doesn't even have one now (he just THINKS he does) because he cannot spray anymore. Something responsible for the spraying process has been removed.
Try as he might, he just can't do it!