She was just trying to do her job =/

I can certainly see how frustrated you might be. When I fly domestic airlines in China they have a "no cellphones" rule, even if it's in airplane mode, even in mid air. Really frustrating I can't work on passing blossom blast level 391 mid air but rules are rules and the FAs sure as hell made sure I knew them before tearing my phone from my hands. But alas, if I fly their airline, I must abide by their rules,regardless of how silly/serious they may seem.

Of course the "should" language is somewhat ambiguous, but I interpret it as if it weighs above 2lbs you should put it away. To avoid confusion, perhaps write a letter to Delta and ask them to clarify their rules.