pick pockets are the most problem
this way they can get a good look at everybody and see who they know
or who matches the profile. apparently only the foreginers look alike

It's mostly for show.  People knowing that these checks are there are less likley to do bad things.  The appearance of control.

A chinese friend told me its because Hu Jintao's son is in charge of the company that sells those scanners.

"Hu Haifeng is a Chinese businessman and the son of Chinese President Hu Jintao. Hu Haifeng was the president of Nuctech, a Tsinghua University-owned company created in the late 1990s to make large scanners for shipping, trucking containers and railway cars, as well as luggage scanners and metal detectors for airports.

After Hu became president of the company, it was granted a near monopoly by the central authorities on the lucrative market for selling security equipment to airports in the PRC. It now accounts for roughly 90% of the domestic market. Chinese investment in airport security has risen sharply after the September 11 attacks"

http://en.wikipedia.org/wiki/Hu_Haifeng

It's all just for show. 90% of the time, they don't even look at the screen. I carried a massive wine opener and swiss army knife in my bag once for almost a week (brought it to a party and forgot to take it out). Took the subway at least 10 times that week and it didn't get spotted once. It's a waste of time and just holds up everything. If a terrorist wants to hurt people on the subway, he/she is just going to smuggle the dangerous item under their jacket.

Actually, the started to install all the security check equipment for the purpose of the Olympic Games, then kept them for the Shanghai Expo, then for the Asian games, and then now they keep them for.......fun.....
In Shenzhen, they discovered that the equipment wasn't approve by the authority in charge and that they were dangerous for children and pregnant women because they emitted some radiation levels.
So don't worry about them......they are just for fun, take it as a game. They will never prevent anything to happen. Moreover, putting your belongings (mainly bags) is something random, some needs to do it, some doesn't need to do it. Me with my child, I need to pass my bag through it (in case I have some dangerous diapers), but a white collar guy won't need it.....

I have been through many different security X-ray machines in my travels in China. I have seen the checker fast a sleep a few times. Many times I have a can of shaving cream with me and it was never a problem until recently in Bejing where I was called out by the security and asked to open my bag. The guy tried to tell me(apparently) that I could not take it on the train. I played charades with him for a while trying to show him what it was. He kept shaking his head until my girlfriend saw what was going on and come to my rescue. Eventually he said that I could bring it with me this time but not to do it again. I still bring it and have had no problems since.

So based on my experience they are mainly for show. 

Job creation.

The first time I saw a Chinese person ignore the security guy and keep walking was the last day I went through the check . Since then I have just walked through every time. And so do most of the people in Shanghai.

Yep, they have security checks. They snagged my little screw driver thingee (a key chain device). Geez, they are getting as anal as the US. Monkey see, Monkey do! How I long for the days when the screener was asleep with his feet up on the puter!

Like the TSA..yep fingernail clippers! Step back boys..I have fingernail clippers. Pull those pieces! Im getting ready to snip you to death!

Those are such a waste of time. I don't know what it's like in other cities but it's so inefficient and pointless in Beijing. If you walk straight in with a backpack, most of the time they don't even stop you. The other day a girl had a backpack and instead of asking her to put it through the x-ray machine, he just felt the weight of the bag (he didn't even look inside!) and let her through. Seriously! Tang-Peter's theory is very interesting and makes a lot of sense.

It could be that China has a lot of things that are made to look like something else. I know they had a lighter disguised as a pack of gum and they also have lighters that are disguised as actual guns. Some things aren't prohibited on the train the same goes with airplanes. Your not supposed to have lighters but since a lot of stuff is disguised to look like something else in China they have to be especially cautious, and they are definitely aware of this.

I was told that they were in kept in places in these cities because of a fear tha the violence and bombings and riots in a certain western province by a certain group of non-Chinese might spill over into other areas of the country or take on a turn like what happened in a certain mideastern country that consists of one county and various other countries occupied by it. I have actually had it seriously explained to me like this.