The only place in china I saw the WC was clean, was run by foreigners. I saw the "owner" by himself cleaning the pipes and sewage with VAP cleaner and all the stuff. Just this one place. And the draft beer is gorgeous there ))

It is because of the excrement ease. Smell stimulates body functions needed for 'unload'.

Smell in toilets is also present to assure user, that many people before him/her were using the same spot. Similar as 'my neighbour has an IPad, I must have one too'.

Because things that take effort in China are not worth it.

In China, toilets are expected to smell. What else would it be if it didn't smell? This is but a small inconvenience you will face. I'm sure you've already gone through most of the nasty habits here.

I guess they simply have different standards for cleanliness...or maybe they consider it as a non-income generating activity.

That's a gumby of a question Tarzan!

I was in a restaurant the other day. On the wall was a notice saying something like "I clean here every hour, if it inconveniences you I am sorry" Then below hung a sheet of paper where you could see times of cleaning noted down. Despite frequent cleaning "attempts" the place was still a shithole. 

Go to "Muslim Restaurant" for clean toilets.