They can... And they do... There is a setting if you use Parental control that will send an email once a week to an adress you supply that will contain an Playtime report for an account you have selected... I accidently managed to switch that on on one of my old accounts.
So yeah, they are tracking your playtime.
^^ This
My youngest son is playing under PC and I receive a weekly overview of his daily WoW activity.
Interesting. I guess I should reduce my 14 hour botting sprees.
If you have been playing 14 hours a day since you started how can they prove whether its boted or live play. I've been playing Since Vanilla on and off and i afk for hours at a time while my toon sits in a city. it would be a very unreliable source if Blizz banned based on daily time played.