so i got pick pocketing high enough that i can swipe the fusion cores out of unsuspecting power armor, this i acknowledge is stealing...
the game however doesn't differentiate between the core i just took and the other cores i've got in my inventory.

the enemy gets ejected from the power armor, and you can kill them and take their stuff like normal. i acknowledge this as Loot

however getting into the now vacant and owner-less power armor Frame is considered stealing? WTF?

i can loot the perfectly good power armor frame of it's armor accessories and place them on a frame i found or bought and nobody says a word.
but if i try getting into the frame of the guy whom i pick pocketed then suddenly i'm stealing and Piper hates me for it.

additionally i can command Piper into the "stolen" power armor frame just fine and no one seems to say a word. however if i get in it while piper is around i get shamed...

Semantically, Looting and theft are the same thing. Both involve taking things that are not your property.

The person you ejected from the Power armor... were they naturally hostile towards you?

This distinction is one of the most glaring oversights in the entire game.

Taking things from raiders, dead or alive, should NEVER be considered stealing. It gets extremely silly, too:

-Pickpocket a living raider? THEIF! FOR SHAME! BOO, HISS!

-Shoot him or her in the back of the head, kick the body around, THEN take their stuff? MY HERO! WAY TO GO, CHAMP! HUZZAH! Just don't touch their power armor.

Which is absurd. What, I suppose they bought that PA from Arturro or Rowdy, 100% legit?

EDIT: If you're sneaky enough, and can kill EVERYONE in the cell before anyone gets in the PA, it isn't marked as stolen (I pulled this off at the Gunner's Highway Interchange base northeast of the glowing sea, and at the Ration Stockpile south of Sunshine Tidings).