Since when did it become rude to simply talk? As you said, they were not yelling. There is no rule, formal or otherwise, against speaking to one's companion. If it bothers you, you have the option of bringing earplugs. I am a person who likes peace and quiet, and knowing that I am unlikely to find that on any form of public transportation, I bring earplugs for every flight and sometimes on train trips, too. (By the way, I hope you never take trains, because there almost everyone talks nonstop!)