I think the reason is balance, BUT FOR YOUR SAFETY, not theirs. And Neon makes that clear here.
It's not fair to you, the guy who wants to commit crime. Even though that seems contrary, right?
But ICly and OOCly, the OTA are more powerful than modern day soldiers (one of the reasons the Combine WON against EVERY military on earth).
So they'd also be more powerful than some rando rebel. The only time my Anti-Citizen was allowed to kill an OTA was because they were isolated, had surprise, and backup.
And If OTA are counted to checkcrime, then that means, like Neon said, you may not have 4 CP and 2 OTA, but 6 OTA. And they know exactly where you are because of cameras... and they can get to you faster than CPs because they are modified and robotic humans... and their first method of getting you to stop is not to yell at you, whack you with a stunbaton or shoot you in the leg with a pistol...But lethal force with a fully automatic alien machine gun.
So you /checkcrime to try and get away with something. 15 seconds later your well-developed char is PK'd after a pulse rifle round goes through your skull, because OTA don't adhere to the same MILD escalation of force as CPs (and as PK from CP rules would need to apply if OTAs are being counted with CP for checkcrime).