It doesn't even matter if some kid that's making ten bucks an hour really is, as you suggest, taking the meat grinder completely apart at least once every day and completely sterilizing every piece of the machinery.And that is why, I believe, an establishment that grinds its' meat fresh on site can cook it more rare. The interior has not been exposed.
The surface of the meat being ground up has been exposed, and carries biological contaminants including quite likely several strains of E.Coli. Once that meat is ground up, that bacteria is spread throughout the food.
And while they can put additives in which would remain pink (so you can't really judge purely on colour), beef on its own will not be pink after reaching a minimum temperature of 71 deg.