Why should we not rid ourselves of aliens who do not share our ideals by pledging allegiance to our Constitution upholding those values?

This seems dangerously close to totalitarianism, an idea tried in Europe c. 1940s, which produced very unfortunate results.  How do you propse getting rid of them?  Who is to decide who is not upholding our values? 

Because we will never have interplanetary peace if we did.

we shouldnt