I think you may have touched on one of the fundamental contraversies in political science: Engagement or isolation? When there's a regime doing thing you don't like, do you oppose and undermine it; contain and isolate it; or engage it and hope to exert a beneficial influence? If the USA were truly totally opportunistic, I don't think it would pressure Turkey, China, and Egypt, to pick some egregious offenders, on human rights as they do. (The case of Turkey is an especially striking argument for engagement: When the EU holds out hope of membership, it moves to clean up its act; when it doesn't, Turkey backslides.)

Another problem is that a lot of our foreign policy is hostage to special interest groups. The Anti-Castro Cubans are one of the most powerful examples of this, but hardly the only one; a strong Armenian lobby in the USA has hampered aid and involvement in Azerbaijan--with its tremendous underexploited reserves of oil--for instance. Domestic political considerations like these also help prevent foreign policy from being as consistent or as rights-based as we'd like it to be.