There’s not unwritten rule, it’s a simple matter of logistics, if you’re keeping control of your car cops can’t tell if you’re breaking one of the rules that exists to force you to keep control of your car. They can only tell if you’re NOT keeping control of your car, at which point they pull you over and figure out why and give you the appropriate ticket. That’s how all traffic laws worked and have worked and always will work.
No, it's an unwritten rule. The written rule says, "No texting while driving."
The unwritten rule says, "We can't really monitor the inside of your car, so just don't drive badly and we won't pull you over for texting while driving."
Note that what that really means is: "We can't really monitor the inside of your car yet."
There have been idiot legislators that have suggested linking into the onboard computers in cars to see if people have been speeding. How long until one of them suggests that since web cams are so cheap and most cars have onboard computers, lets record people while they drive?