Sounds like there would be an opportunity for TB mules to do some favors for our overseas brethren. I don't know if customs requires you to return with an instrument that you brought over, but I don't think there'd be an issue if it was a "gift" (unless you are so tightly monitored by the SS you have declare a gift from a friend).
I think they take the VAT pretty seriously in Europe. I don't know how they'd check, but they probably have some sort of declaration you have to make. You could probably get away with it once, though. I'd be game to try.
On the OP's question, Americans often have more disposable income to spend on stuff, consumption is not taxed as heavily here, and credit and products are readily available.
Americans are also more obsessed with the status their belongings convey. If you want to see the difference, watch the U.K. version of Top Gear. The hosts there can afford to buy Ferraris and Porsches, but they go on national television dressed as if they shopped at thrift store. I just don't think you'd see that in America.
Personally, I only buy American, and preferably made in California (Fender, Music Man, Taylor, Santa Cruz Guitars, etc.) I'm of course biased toward California because it's my home, which is completely emotional and not rational.
But the labor and environmental standards in California are some of the strictest in the world and we have decent wood importation rules here, so I know my money isn't going toward exploiting workers, polluting the air or buying illegally logged wood. When Gibson got raided by the Feds for illegal wood, I personally stopped looking at their products for a while. They're probably back into compliance now.
Since basses are made just about the same way everywhere, we can assume the difference in price reflects international cost differences for labor (which, aside from some macroeconomic monetary theory crap, mainly flows from how well paid and protected workers are) and from allowing pollution and the use of questionably obtained wood.
I don't care to support that if I have a choice. If I can't afford to buy from a country where people and the planet are treated well, then I can't afford it. I would have no problem buying an instrument from the U.K., Germany and Japan, if I dug them.
But all of this is a luxury that I enjoy living in America. I have nothing against people who buy foreign instruments if it's all they can afford. I do encourage them to buy second hand though, because it saves you money and it's better for the planet to keep an old axe going than to cause a new one to be made.