Libertarian columnist Vox Day offers an explanation that might help:
The "freedom" espoused by the utopians should never be confused with the unalienable freedoms that are the American birthright, however. It is no accident that despite the fact that they speak of an American empire, the quasi-democratic systems that result from American military invasions and occupations are inevitably free of the not only the checks and balances of the American Constitution, but also a good part of the American Bill of Rights. [...]
The reason that advocates of utopian empire are inherent traitors to the United States and enemies of its Constitution is because without respect for national sovereignty and self-determination, the United States itself has no raison d'etre. The protections of its constitution are nil and its unalienable rights are void if they are in conflict with the wishes of the utopians. In the same way that neither the Serbs nor the Kurds are permitted the right of self-determination under this utopian scheme, Americans are denied the very rights that they are supposed to be guaranteed. [...]
And because it offers the promise of freedom while delivering its opposite, the neocon's utopian concept of empire is doomed to failure by its inherent inconsistencies. The World Democratic Revolution is no more tenable than the World Communist Revolution, and like its intellectual parent, will eventually collapse into totalitarian tyranny. The particular danger for the United States is that following the tradition of imperial overstretch, its abuse as the utopians' primary weapon will cause the remnants of its constitutional system to break down as well.
Quoted from "On global empire" at Vox Popoli.