In my experience and talks here, overtime work is compulsery, and may not be paid for at all.  You are hired to do a job, and paid for it, no matter how many hours it does take you to do it.  Yet, I also know that extra pay is mandatory by Law, but seldom done.  I also know of a case where a State run College was taken to local Court here for forcing some employees to work during lunch hours and holidays with no extra compensation, a group of about 20 of them (my GF included), and each was awarded over 100,000 Rmb accumulated over a number of years.  And then, they were all fired afterwards !.  And in theory, this is supposed to be a worker's paradise.

Or it could be that they are filling in for other employees. I had to do that on several occasions. Most people like overtime work because they get paid for overtime work. I've really never seen anybody complain about overtime work. Well not in the states at least people there ask for overtime work because it puts extra cash in their pockets. The same thing goes for China if you work overtime here they are supposed to pay you extra.