Well, I guess it is until it isn't, you know?
So far, I've enjoyed it with no ill effects. But there's still time for calamity to strike. I'd take reasonable precautions with any street food. If it looks okay, I'd risk it. Your mileage may vary.
Ken

Well. if the soybean you get from the street vendors tastes watery, then the reason may be the vendors mix much more water to the soy milk, and this kind of product is nor harmful to people's health.
However, if the soybean milk is thick but the price is cheap, you should be cautious. It is reported that some unscrupulous vendors produce fake soybean milk with a kind of powder in order to save the production cost.

Home-made is the best, haha

I think you have to careful with the soy drinks you get from street vendors. Why? Because if you look carefully, most of those cheap plastic cups have no mention of ingredients, source of origin, where it's produced/packaged etc. They are not screened for bacteria or other harmful substances and they haven't passed any health tests. While you may not get sick from the immediatly, you can be sure that they use unboiled tap water for producing it, and they certainly don't clean out their tanks every day. The long-term benefits of drinking unboiled tap water in China aren't clear, but with so many metals and chemicals seeping in the water system, I guarantee you it can't be good for you in the long-term.

Njord is enterily right, Redaria. Although it would not necesarly be a bad thing the be moar suspicious. By losing weight, you're certain not to turn into a mythical Irish land whale, Redaria!

And quality greatly varies from vendor to vendor. It's usually my Chinese girlfriend that checks the quality, maybe like daydayup said. Usually, when it's good, she will buy, or else she'll make an angry face and spit on the ground as a sign of disapproval.

It works, because I have never been sick on soymilk!

I would never buy it from a vender just because it isn't cooked/hot. making soy milk is easy if you have a machine and it will pay for itself very quickly.
1. buy soybeans and soak over night/few hours
2. drain and rinse
3. put in machine, which is just a grinder that heats it all up and stirs the beans into the water
4. let it cool down.
5. drink.
6. remember how much better it was when you could drink milk without fear.
7. *sigh*