I'd imagine a Rugby player isn't used to armored human missiles making tackles that would be suicide without a helmet. That whole highlight reel was guys slapping at him with their hands.
Of course, so were those punt returns, heh. "special" teams indeed. They give you pads for a reason, go out there and hit somebody.
I imagine the average kicker in the NFL is a more effective tackler than most Rugby players, if only because of helmets vs self preservation.
But I guess there are a few toothless guys with shattered skulls and fused vertebrae that just don't give a fuck.
Also, it helps being the only black guy on the field.