...I think the only thing this man needs to be aware of is whether the person he is trying to chat with is receptive. If not he should stop.
I think this part is the key. In every region of every country in the world there are different cultural norms. While the best possible thing to do would be to learn as much about those norms as you can, it might be impossible to know. In my life, I've lived in New England, the West Coast, the Midwest and the South and things are completely different in each of those places.
- In New England, where I lived at least, small talk would've been met with a concerned and annoyed stare. Anything personal or private (like a bus rider carrying crutches for example) would be rude to mention or point out.
- On the West Coast (again, just where I lived), small talk about basic topics was okay, provided the person didn't seem to be in a hurry. Personal or private topics were still off limits.
- In the Midwest, small talk was constant and encouraged. If someone attempted to board a bus or other transport with crutches and you didn't assist, offer assistance or at least ask if they were all right, it would be rude. (Presuming you were standing closer than anyone else and thus most likely to be able to assist).
- In the South, the cities have had a similar culture to New England so far, with everyone going about their business and not being interested in small talk; whereas the country and suburbs have been closer to the Midwest, with the shunning of small talk seeming worse than engaging in it.
I know not everyone's experiences are the same, but those have been mine. To me that says that knowing exactly how small talk will be seen is impossible and that it isn't rude to engage in it even if the culture isn't receptive to it. UNLESS, you continue to do so after being informed or made aware that it isn't a good idea in the area. For me, the mistake the American made in the OPs story wasn't making small talk at first, it was continuing to do so on the assumption that he would be off the hook for being from somewhere else.