That is why that option was moved to the attack menu.
My 2-cents comments on the matter.
If the mod has a large TU for throwing, I always considered the priming to be part of this action. I always saw the priming of grenades that have a timer to be not the grenade pin but an additional safety measure, such as a lock or arming button. Then when pre-priming became available all made sense to me.
In my mind, the grenades in a pocket that were accidentally dropped and later exploded, they hit the ground without the restraint of their last safety element. I imagined that to be a sleeve of sorts in the pocket or backpack holding the grenade lever or similar that lost its grip on the grenade upon impact. A soldier would grab a pre-primed grenade from the sleeve into his/her hand for throwing. I consider the sleeve to be an invisible item that adds to the encumbrance of the grenade in the overall weight management of the game.
Are the priming or throwing costs reduced somewhat for balancing purposes?
(I haven't played K1 version yet, so I may be writing non-sense, apologies if so)