Its the US high command's stupidity.
Go in. Capture weapons and ammo. Destroy said weapons and ammo.
After forming new police force and military, spend millions of dollars buying new ammo and guns that are the same ones you wasted time and gas destroying.
Remember the piles of burning weapons? Alot of AK rifles, many in VG condition (some pics may still be on dragunov.net). So we destroy the AK rifles, and then... buy the same types of guns! The difference here is that we have to pay now.
The US could have gotten rich if they expanded the NFA to allow a few new full autos in, as well as turning the rest of the guns into parts kits and also redistributing/selling the ammo and mags in the US.