There were two black heads in the Star Wars 2011 advent calendar and neither had a face drawn. Is that because there was no face necessary since it was going to be covered with a mask anyway or is there a rule that the faces only go on the yellow heads?
They made them solid black so there are no alignment issues when veiwing the masked minifigures. By leaving them blank LEGO ensured the eye slots always appear black. Side note: The only Stormtroopers seen without helmets in the StarWars Movies were Han and Luke in disguise, therefore Stormtroopers are faceless.
The latest Star Wars sets (for example 9489 - Endor Rebel Trooper & Imperial Trooper Battle Pack) do have faces printed on the black Storm Trooper heads, which doesn't work all that well with the Scout Trooper, unless you put it on backwards:
However, looking at them, you can clearly see there are issues with printing on black plastic - the faces aren't quite the right colours (although I'll need to work on some better photos for this to really show).
Edit to add:
Noticed while rebuilding the set the other day that Jango Fett's head was also black with a printed face, released in 2002:
Well, they don't print them because they don't know how are their faces. For example, have you ever seen a stormtrooper's face? No, but that's weird, the stormtroopers were suposed to be clones. Anyway, they don't print them because they or we haven't ever seen a stormtrooper, scoutrooper, or whatever unmasked.