Neither is human sh*t, semen, urine, or phlegm, but I would not expect an Apple service tech to have to clean those off either.
But dangerous to what? If it causes dust to cake and cooling to fail, is that no dangerous to the computer? What is the resistance of a layer of tar on a motherboard? How does it not harm a hard disk drive when it clogs the air filters on the breather holes? How does it not harm an optical drive when it is deposited onto the optics of the head?
Hold it right there! Why should my Applecare subscription, and the subscription of all of the non-smokers, be paid to Apple technicians to strip smokers' systems down to the bare boards to do hazmat cleanups?
So how do you "educate" a worker so that they don't gag from the smell? How do you educate them so that they don't say "F*ck this! I'm going to work at a data center."? No amount of educating will prevent the revulsion that many techs would feel in that situation.