> is strange that when people are pro-american, or pro-any-ingroup, nobody asks for justification

Being pro a group can be strictly positive sum - wanting to lift that group up, likely because you consider yourself part of it or on the same team. It's possible your intentions are bad, but they certainly don't have to be.

Being anti a specific group is inherently negative. Perhaps they deserve it, but that requires justification in a way simply being positive does not.