It's usually because they don't allow localities to add sales taxes. Here in Japan, sales tax is 10%, everywhere (except for groceries, which is 8%). You don'

It's usually because they don't allow localities to add sales taxes. Here in Japan, sales tax is 10%, everywhere (except for groceries, which is 8%). You don't have to worry about an extra 0.5% from some municipality; it's the same everywhere. The US gives way too much control to local governments.