well, they dont french kiss the thing do they? people put their hands in their mouth all the time.
Study up on your germ prevention! ;P
Adults and children (excluding babies since they don't know better) shouldn't be putting their hands in their mouths anyway unless they've washed them first.
Btw, I wasn't serious about nuking the black rock due to the germs when I said it. That would be overkill but I personally wouldn't kiss that black rock nor the Blarney Stone due to the germ factor.