The benevolent United Federation of Planets - which we are told repeatedly in all incarnations of Star Trek has ideals of peace and justice for all - not only sponsored this deliberate treaty violation and incursion into sovereign Romulan space to perform a blatant act of military espionage, but also ordered Kirk and Spock to be complicit in a deliberate cover-up aimed at providing deniability to the Federation in the event that they were caught.
And then... when the mission turns out to be a success and they escape with a functioning Romulan cloaking device, the Federation never uses the stolen technology in any of its own starships!
One wonders what happened to the cloaking device that the unwitting pawns of Kirk and Spock stole. Maybe it ended up in a secret government warehouse buried beneath The Pentagon on Earth... or maybe it got into the hands of whatever shady conspiracy ordered this mission in the first place, and they end up using it for their own nefarious purposes in the future.
Medical log, Stardate 5027.3. Captain Kirk has gone insane - again - taking us into Romulan space, without Starfleet orders.
Tal: You're surrounded, in 3D space, by just three Klingon ships that we Romulans are using for no apparent reason!
Commander: I am the Romulan commander.
Kirk: A woman!
Commander: Your first officer will wait outside.
Kirk: Yes, please!
Commander: You're here to steal our ship's cloaking device! Admit it! You want to uncloak us!
Kirk: The thought had crossed my mind…
Spock: Captain Kirk ordered us into Romulan space on his own authority. He is insane with power.
Kirk: Traitor! I'm not insane!
Kirk: I'm not insane!!!
Kirk: I'll kill you, Spock!
Spock: Oops, I reflexively used the Vulcan Death Grip.
McCoy: He's not really dead. It was just a nerve pinch.
Kirk: Spock and I had secret orders to establish deniability for this Federation black ops mission.
Kirk: Bones, perform plastic surgery to make me look like a Romulan.
McCoy: Some make up and plastic ears could-
Kirk: I'll infiltrate their ship and steal the cloaking device while Spock and their commander make weird alien love with their fingers.
Kirk: Scotty, install this cloaking device! You've got 15 minutes!
Scotty: Cap'n, it'll need coils and stuff installed all over the hull!
Caption: Surprisingly, it doesn't!
Kirk: Beam Spock back. Cloak the ship! Warp 9, Mr Sulu!
Sulu: Aye, captain!
Caption: She appeared with Spock
Commander: You may have stolen our cloaking technology, Kirk—
Kirk: Yes, but we'll never actually use it on Federation ships!