Because Sam and Dean are so completely in each other's pockets that neither one of them has the slightest bit of shame around each other anymore. If that had been pretty much any other two people from TV I would've been hiding my face.
ETA: Also? Sam looks completely five there in that scene, with that big, giggly smile of his. And the way Dean just kinda nods to Sam for the "WANTED!" backup makes you just know they've been doing sing-alongs since Sammy could make word-sounding noises.
*Does NOT meta about Sammy singing back-up*
