I have often walked in on political discussions at my local bodega. Because I don't look hispanic they went on thinking I couldn't understand the language. Sometimes I leave without saying anything. A few times though, I couldn't help myself when they were ragging on the country. I would let them ramble on until I was ready to leave and then in perfect Spanish, I would tell them what to stick in those dark private places and what they could do with themselves. After they swam back where they came from of course.
I hope I'm not missing any sarcasm here, but isn't the intentional use of another language to exclude others even WORSE than inadvertently speaking another language?
To me that kind of behavior is the equivalent of whispering behind cupped hands when someone else is in the room. In a business setting, I would take that kind of behavior as very rude. If you want to keep a conversation private, you should find a secluded area or wait to continue the conversation when you are alone.
I don't consider it rude if people are speaking in another language simply because it's much easier for them. But if it's done to keep other people out of the conversation, I think it is very rude. Of course, since I'm not bilingual I guess I'll never really know if I should be offended :)