It should be illegal, but I do think you might just be living close to some people who really love trucks. 5k is not a lot across Europe, popular models sell 10x that.
If the US government wants to give its soldiers perks, they can rent or loan them a local car. Probably cheaper all round than flying/shipping in their financed Dodge RAM anyway.
Then again, American personnel being arseholes to the locals is well established from Okinawa to Croughton so it's probably endorsed as a power thing.
They’re clustered around areas of idiots with means. I’m nowhere near a us military base but there’s a bunch of these where I live, including two or three owned at houses I pass by on my way to work.