What's the rationale for why living subjects get a test to resist Levitate but Drones would not. It's just quirk of semantics/word choice in the rule, or is there an underlying logic to Levitate's exception in allowing a resistance test on a spell that uses a (normally) unopposed success test?
Take the example of an anthropomorphic drone holding a gun, and a human holding a gun. If enough levels of Lifelike Features were present, you potentially wouldn't even be able to tell which was which! But if you randomly select the gun in the human's hand, the human gets to roll BOD + STR to hold on to it. Why shouldn't the drone, if you had tried to levitate that gun instead?
The rationale is because it says that black and white in the rules?
LEVITATE
(PHYSICAL)
Type: P Range: LOS Duration: S Drain: F – 2
Levitate allows you to telekinetically lift a person or object and move it around. You have to beat a threshold equal to the subject’s mass divided by 200 kilograms, rounded up. The subject of the spell can be moved anywhere in your line of sight at a movement rate equal to the spell’s Force in meters per Combat Turn.
If you’re trying to levitate an item held by a living being, or levitate an unwilling living being, that being can defend against the Spellcasting Test with Strength + Body. You can use this spell to levitate yourself, if you need a view from a higher perspective or desire some parachute-free sky diving.
So, actually, lifting the guy's gun out of his holster would be just the threshold test. By the text of the spell, if it isn't a living creature holding on to the thing, there's no resist roll, so no spell defense. You could make an argument for a drone with an anthropomorphic arm holding something, or a drone with jaws biting something getting a test to hold on to the item, I guess, and a reasonable DM would probably agree with you. However, that is an extremely niche ruling, that only applies to a small subset of drones under an even smaller subset of conditions.
Note that you can't use this to disarm a drone's mounted weapons, since those are usually locked in, not just resting in a holster or the like.
If you're asking for an in-game reason, then just chalk it up to the complexities of how mana interacts with living creatures, or something like that. At some point, you have to just shrug, and say, "Because that's the way it works." Especially when dealing with magic, since Magic doesn't always play by rules we mortals entirely understand.
My question with the drone would be "Is it being rigged or is it in auto mode?".
It doesn't matter. Anything installed on a drone can't be lifted off. Anything an anthro drone is holding would be in the same grey area regardless of it being rigged or dog brain. Lifting a drone with the spell is no different from lifting a car (and the people in it), except that you would need to factor in the weight of the people in the car as well as the weight of the car itself to the hits needed. Spell defense would not come into play, but dispelling could.