That could be used to justify banning drones in general, or banning all drones which aren't radio controlled (not that those are being used domestically). And "it can be used for war" is a bit silly in a country where you can buy guns at the grocery store. Not to mention that cars can be very effective weapons as well, and those haven't been banned yet.

The far more likely explanation is that they just don't want people filming them. They can't legally stop someone with a cellphone from filming them, but that hasn't stopped them from using up-to-lethal force against observers. On the other hand, you can't exactly beat a flying drone into submission, so the obvious move is to observe using drones instead.

Luckily for ICE the FAA already has the mechanics in place to criminalize flying drones in certain places, so with their magic "no drones anywhere we operate" NOTAM they can now punish observers with a year of jail time.