Just a thought... not to be awkward, but is the fact that you are a North American born/ native English speaker stamped on your forehead? If not, then the only thing the agent had to go by was your looks, and you know as well as I do that people are reacted to based on their looks. I know that my ability to speak a certain language (English, German etc.) is not stamped on my forehead and therefore, since I manage to look like an obvious tourist, EVERY time I am in Europe I am asked almost once a day if I speak English because the other party wants to be able to communicate with me in the easiest manner possible. Just like if I were in Quebec... how can the other party just assume I speak French? I would never make such an assumption!

In this case the lady may have been trying to ensure that you were able to read English because many Asian travelers don't (conversely, it would be like assuming I can read Kanji or Korean which I don't), and often require a translator or language assistance -- that's a fact and not a generalization and for instances exactly like these when you have 000s of people speaking in tons of tongues, they have language assistance available at airports. Now if she was rude or condescending, that's a different issue and in that case, yes, she was being offensive.

So ultimately only you can answer the question which no one here can... Was she being condescending or was she genuinely trying to help?