I am rather grateful to such people, at least when I'm not right next to them, as I normally talk to myself when I walk, and now other people no longer treat me as crazy. It makes it much easier to work out technical problems when you can talk out loud.

The reason cell-phone users shout is actually technical: cell phones don't provide any side tone, the 20% or so of your own output that a land-line phone feeds back to your ear. Since you don't hear yourself properly, you unconsciously raise your voice. "Talking to a cellphone is like talking to a hairbrush."