Per your question about cash games-
Some rooms are fine with it as long as you don't slow down the action. Other rooms would rather not be bothered by deciding if you're slowing down action so they just don't allow them. It's essentially just whatever the management of that room prefers.
For tournaments, I believe there is a TDA rule in place to not allow cells phones to be used during a hand.

This is something that's evolving. Used to be a no-no to ever use the phone at the table. This was a carry over, I think, from the pit. I haven't played any pit games in a long time but last time I did I absent-mindedly looked at my cell phone and was warned about it.

As cell phones have gotten more or more advance and are used for more and more things, most rooms loosened up the rules about them. A lot of phones are used as players' music player. Texting seems to be ok as long as you don't have a hand.

I wrote about increasing cell phone use in the Vegas rooms in a recent Ante Up column, you may find it of interest, it's at:

http://www.anteupmagazine.com/nevada/news/cell-phones-at-the-poker-table-are-here-to-stay.htm

To comment on the 2nd half of your question, I think the difference for tourneys is two-fold:
1) It's easier for collusion to be effective in a tourney,
2) There's theoretically more money at stake. Obviously both cash games and tourneys can be small or large, but consider that first-prize for a $100 buy-in tourney is going to be thousands of dollars, while it's very unlikely that any player in a low-stakes cash game is going to walk away with thousands.

I thought it had something to do with slowing the game down, and potential for cheating like using odds calculators. But it probably also evolved from the old-school restriction of using electronic devices at gaming tables. Remember how viciously they used to hunt down people in sportsbooks? They used to come out of the woodwork like snipers haha