Diificult issue. Granted, the cargo is the property of the owner, and has the right to do with it what they want. However, the attacker is risking potential retaliation, and therfore his risk should have a reward. Personally, I find it lame that someone would delete their cargo, because, frankly, if I did not take proper precautions I deserve to lose it. Also, though, if I can deny my foe their reward in retaliation for them attacking me, I at least have the satisfaction of "flipping them the bird".
Here is a possible concept that might work as a compromise. If a bot has been targeted, is under attack, and taking damage, then some of it's storage access might also take damage. Other than ammo, for obvious reason, maybe the way to work this is that whatever percentage of damage that bot has, that percentage of cargo cannot be accessed for deletion or use.
This still leaves the bot under attack the ablity to deny it's attackers a portion of his/her cargo, yet still leave some to drop. Depending on how hard and fast you are able to apply damage, will determine how much of the cargo cannot be deleted.
EDIT: Perhaps this should be worked starting with targeting. If a bot gets targeted, 25% of it's cargo immediately cannot be deleted. As damage is applied, for every percent of it's total hp damage, that percent of the cargo, in addition to the 25%, is not deletable.
In the gods we trust, all others bring data!