Delta In-flight Information about Portable Electronic Devices

I posted Delta's portable electronic device rules as an example, though I'm guessing these rules are the same among all airlines.

For the "Devices You Can't Use," do these actually interfere with safe operation of the plane?

For the "Devices You Can Sometimes Use" (with the exception of cell phones) what is the purpose of this? It appears that they wouldn't interfere with the operation of the aircraft if it is sometimes okay to use them. It looks like it is okay to use them for the cruise phase, but not for T/O and landing. I'm thinking that it must be because airlines do not want passengers to be watching DVD's with headphones in their ears (for example) during the safety demonstration by the FA's and during the critical phases of flight in case of an emergency. Is that correct or is there another reason?

Is the cell phone rule there to make sure that passengers do not violate FCC regulations?