That's actually the primary reason. They want passengers to have as few distractions as possible while the safety presentation is reviewed, and also in the event of an emergency. This is also why they have no qualms using them after landing.

If cell phones were a major worry with the FAA to interfere with navigation equipment, they'd be banned completely. The interference you hear when your phone rings is because it's in very close proximity to the device in question (speakers on an appliance usually).

The FCC is who you need to worry about if you're using your phone in the air. They (along with the mobile phone provider) can, and have been known to, follow up on cases of cell phones attaching to multiple towers - which is not something that happens on the ground where they're supposed to be used.