It's kinda like taking a book on an airplane with you so your seatmate will be less likely to strike up a conversation when you aren't in the mood. When your eyes & attention are on something (book, knitting, crochet, iPod, ...) people just generally believe it is polite not to interfere and distract your attention.
However, I guess there are always exceptions to the rule I guess. Last fall while knitting a hat on an airplane, the 70ish guy in the seat next to me started talking to me within minutes of the yarn coming out after take off about the hat, the pattern, how long I have been knitting, what was I knitting next, how long would it take to finish the hat, etc. and then this conversation attracted the flight attendant who joined in and talked about her craft hobbies.
At the time I guessed he might have been less likely to comment on the hat if it had not contained the logo of a university during football season (and we did talk about recent and upcoming games and previous seasons/players/coaches while I was knitting too), but perhaps that was just unjustified stereotyping on my part to think that a white haired male would be more likely to start a conversation to talk about football than knitting.