Because a civilian owning a walking nuclear warhead is acceptable by your local government. Yes, tell yourself that.
The easy way is for him to rise to General of the army, or head of all exorcists. Or the Minister of National defence.
No one will give shits if you own nuclear warhead then.
Ok, MC is technically part of the exorcist/army/government. But his rank looks like a private. No private should own nuclear warheads.