Why? The US is unquestionably the largest institutional force that pushes, finances or executes imperialist ventures today. The US bullies all other countries to toe in its line, punishes those who do not, and arbitrarily attacks any country deemed to stand in the way of its imperial ambition. Unlike, say the x-USSR, whose imperialist ventures in central Asia or Eastern Europe were carried by the ruling elites with little popular support, the US brand of imperialism has huge popular domestic support. What is more, all political institutions, from Congress to political parties, have totally capitulated to the national "security," military industrial complex and the imperial presidency that wage wars as they see fit with the blatant disregard for the constitution and international law.
The US is the single institutional force responsible for vast majority of all imperialist activities in today's world. No one else comes even near. Why then opposing this institution of imperialist expansion is anti-imperialism of fools? Would you also say that opposing the Nazi Germany was anti-fascism of fools?
Wojtek