Cell phones looking for signal will interfere with our VHF communication, similar like holding a phone next to a loudspeaker. So that can be annoying.
Flight safe mode on smaller devices is allowed in our company, but only recently. Before that, we were required by law to make you switch them off. A big laptop during take off and landing is a security risk in case of evacuation, that's why you have to put it away. I think that in most cases, it's more a concern for lawyers and liability than actual risk, as are many other safety requirements (yellow fluorescent jackets everywhere anyone?)
