Seems like anywhere here. They don't always enforce the rule.
But if you get caught on the day they decide to. Or you meet the new recruit, then watch out.

@Chingis
Belated - but as you already now know - yes it is technically required, with random enforcement. As noted above - if you get caught AND they decide to enforce - it'll be expensive, not to mention the threat of deportation.

RISK MITIGATION PLAN
IF you elect to go the cheap route AND IF you get caught - apologize profusely for not studying the rules properly and ask if you can write a formal letter of apology (translated into Chinese) acknowledging your guilt, requesting forgiveness for your negligence, and promising to carefully study the laws of China so you can avoid this kind of embarrassment and trouble to the local police in future. This should appease the officials at large and may even avoid the hefty penalty fee. The fee may be arbitrarily reduced proportional to one's grovelling ability and perhaps your astrological sign, I-Ching and FenghShui related things.

This is totally counter to US legal practise where one is taught to adamantly deny guilt and vociferously proclaim innocence until "proven guilty". This will definitely land one a rapid deportation with a return ban proportional to the depth of one's disrespectful attitude...and a maximum fine (where one shall waste away until family and or friends can "bail" you out)...temporary debtor's prison.

China can do lots of irritating things to irritating people.

I have had a large 500rmb per day fine (a couple of months worth) negotiated down to one days fine.
You have to be very nice, my wife was, this fine was paid and was followed up with a parcel of tea and a pack of cigarettes. Since then I have been treated very well and very efficiently.