The same reason you're not allowed/frowned upon to record law enforcement in public. They don't want you to have evidence of their wrong-doings. It's not about the phone per se, it's about the phone's audio/video recording abilities. I'm sure video-cameras aren't allowed either.
EDIT: Wow, those downvotes. My mistake for posting on controversial topics at night.
melling, you're the best. :) I've seen your comments before, I got your number(pun intented). What do you think most people are thinking when they read your reply to my comment?
They're thinking about what percentage of cellphones currently in use have audio and/or video recording abilities.
EDIT: Wow, those downvotes. My mistake for posting on controversial topics at night.
EDIT#2: Spoke too soon... :)