I used to live in Denmark where they (according studies I won't bother digging up now) have both the happiest people on the planet AND pay the highest percentage in tax.
On the contrary. The author seems to be trying only to apply her conclusions to America, so it would be appropriate.
Also, bringing other variables into the regression (societal factors, etc.) would just muddy the waters. If we've got a sample that excludes some variables altogether, it would make sense to use it (with the caveat that we can't know what effect those other dimensions might have).