Perhaps it's because they see the U.S. government as being hypocritical. Claiming the moral ground while expressly going against the Declaration and Constitution.
I believe the Declaration of Independence states:
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."
While not strictly law, it does indicate the moral standing of our forefathers.
I believe the Constitution also guarantees the right to a fair and speedy trial.
If we no longer believe in these, then fine, throw out both documents and have at it. But don't claim moral superiority while your down in the mud.
Of course many injustices have been done in the past like the treatment of the Native Americans and the U.S. citizens of Japanese decent during WWII. There will be plenty more I'm sure.
Injustice happens all the time, but that doesn't mean it should be ignored.