There is a major difference between the ATV with a solid axle and a tractor with the diff lock engaged.
If you were to compare the torque created by each and see what the axle diameter is in relation you'd be surprised how small the tractor axle is. Another factor is the weight put to the ground. ATV's have floatation tires that distribute the weight over a large footprint, therefore making the pressure per sq in verry little in relation to a tractor that weighs many times more and needs traction to perform. Traction variation breaks axles-period.
The diff lock is designed, on any vehicle, to be used in a straight ahead-forward driving motion. If for some reason, you needed more traction due to wheel slip, it is available to divide torque to the ground equally.
On the road, or in a front axle, you would see limited slip differentials to allow slippage before breakage. They work well, but would wear out quickly on a tractor. As little as most actually need a diff lock, it's a simple decision to add the manual lock option to the tractor instead.
My experience on a tractor tells me that the diff lock will remain on (or should) as long as it is needed. Meaning, if you push the peddal down to apply the lock, the traction difference will cause a "bind" so to speak and keep it engaged untill the wheels have equal ground speed. At which time it would unlock, or fall out of engagemnt.