Thats not time theft, at least not if it is within the bounds of the contract. If your contract says the time must average to 40 hours (Which is common) then it isnt time theft as it balances out int he end. Now if that isnt in the contract then it isnt time theft because thats what you agreed to when you were hired, and if you were smart you were sure to negotiate a higher price as a consequence of that.
Now if you choose to negotiate a very poor contract for yourself that still isnt theft, thats you making poor life decisions when accepting a job.