I didn't say Europeans use butane coolant in their cars. They use it in home refrigerators, which might be a greater safety concern. A quick search found:
* R 600a is pure iso-butane.
* And Mixed propane and butane.
R 600a is generally used in household refrigerators in Europe especially, and now starting widely used in developing countries.
One guess might be that since they export cars to the U.S. and the EPA opposes HC coolants, they don't want to double their design effort and parts, plus AC repair shops aren't setup for it. An interesting question is the EPA's motives. Was the "ozone hole" threat real? I understand the widely storied "deformed frogs problem" was found due to pathogens that have always been around and not increased UV radiation.