It would be harder to make a programmer for import vehicles because so much of the power comes from computer tweaking already. It would be more expensive and less effective. Domestic gas engines are less complicated, so computer upgrades can be more effective, and diesel engines are the least complicated, so chips can be very effective.
You do run risks with a chip, though. You have to keep an eye on engine temps, you probably want to go with synthetic oil and change it more frequently, and when you're getting more power, you're losing fuel economy. As someone said above, you have to pay to play.