I think they just don't think about it really. I don't think it is a conscientious choice to do something that might make someone upset, or even that they even really pause and consider it for the half-second that it might take to realize that taking pictures of other people's kids without asking isn't ok. The photo takers, if you notice, are usually the ones without kids of their own, and the touchy feely folks tend to grandparents/older people who have issues with boundaries in general. I think in general the Chinese culture is more communal about children and people do not get up in arms about other people disciplining their children, or offering food to strangers' kids, picking up someone's kid, all stuff that would really ruffle feathers back home.
That stuff I don't mind really, but the picture taking bothers me because it is something that only happens with foreign/mixed kids. Most people don't bust out cameras for Chinese kids, no matter how cute they are. We took Dylan out to the Midi festival this past weekend and people took loads and loads of pictures of him, and I'd say that they probably asked for permission about 50% of the time. If I feel like it I'll block people from taking pictures without permission by getting into the shot or walking away. Dylan does not make much of the picture taking probably because he's grown up with the attention. It is positive for now, but I worry about later on. I don't want him to feel like he's more special than other kids because he's constantly being told how cute/smart/awesome mixed kids are, nor do I want him to feel like an outcast because people are constantly pointing out his differences (even if they mean it in a positive way). So as he gets older we have to figure out how to handle all the extra attention.
George posted while I did, but I don't take pictures of random Chinese kids. Not without asking their parents anyhow. Do most foreigners do this?