The Amish are hardly the only endogamous group in the US. Many small immigrant communities engage in similar behavior, because due to cultural reasons, they tend to marry into the same group, it's only as they integrate into the greater society they become more exogamous. Now, the difference here, is that they remain voluntarily isolated after so many generations and they don't tend to "import" mates from over seas as some smaller immigrant communities might do.
The main difference (to me) is that other groups use genetic testing to prevent diseases in their children. For example, Ashkenazi Jews use genetic testing to avoid having children with Tay-Sachs.[1] But the Amish's luddism causes them to reject these tests, ensuring that some of their children will needlessly suffer and die.
Plenty non-luddite children suffer and die from car accidents and drowning in swimming pools without us much registering a blink. I think it's short sighted of these people not to do the genetic tests as you say, but I get the sense that your desire to judge them for being a weird outgroup is preventing an objective accounting here.
I grant that genetic testing among some of these groups help avoid some of the negative consequences of a small gene pool, but let's not forget that some people in perfectly modern societies make odd choices about preventing disease such as the anti-vaccine movement who are generally well educated and who aren't typically characterized as luddites.
