I think they just want to practice English... I mean, I doubt they're intentionally trying to wind you up Usually people will be happy (and relieved) to lapse back into Chinese when they realise that your Chinese is better than their English, but occasionally they will persist. I don't mind as much if English is the stronger mutual language, but it does get a bit annoying if them using English is hampering communication. I'm thinking less of social situations here and more of formal situations....say, a landlord or bank clerk. There is one lady in my local 7-11 who insists on using English every time I go in, and for some reason it drives me up the wall. Other than that it's mainly just waiters in Mos Burger saying "Welcome" and "Here is your meal, sir", which I don't mind at all.
One of the strangest and most annoying occurrences happened in Mcdonald's last year. I went to the counter and, in Chinese, said I'd like a Big Mac meal (while also pointing at it). The guy looked scared and confused, then turned to his fellow worker and said "我不會講英文!!". He then turned back to me and said, with several pauses and stuttering, "... can't.... speak English.....wait.....". Then I replied with "沒關係，我會講國語!" He then replies instinctively in Chinese, saying "稍等一下", then comes back with the designated English speaker, who says "hello". Wanting to make a point, I repeated "我會講國語", and the new guy then takes my order with the whole exchange in Chinese, the other guy standing there looking confused. Now, you might be thinking that I just have sucky Chinese (quite possibly true!), but I don't think that's why I wasn't understood. I think it was more an assumption on his part that I couldn't speak Chinese, and nothing (even speaking Chinese!) was gonna change that. I mean, even if I was completely mute, I was pointing at the damn Big Mac meal.
I wonder, how good does a Western-looking person's Chinese have to be in order to avoid these sorts of situations?