I just don't get it....

Really?  I just don't get it.  Do you see what this person has hanging from the hitch of the truck?  I see it way more often than I care to...and each time it makes me uncomfortable.  I really don't want to see a scrotum hanging in front of me on the expressway.  Mmmmm, K?!
Is it a secret scrotum society or what?  Am I missing the fun in it?  If so, please explain.  I can't wait to hear the logic behind wanting your truck to "have a pair."