Whether that is a true statement or not ("all" Americans...really?) is beside the point. My point is that when somebody introduces the line "...I on the other hand, hold America to a higher standard than other nations", they are probably a liberal trying to lend cover to their attacks on America. It is an all-purpose line, applicable to any anti-American screed. It allows the individual who loathes America (including those who think America still needs to "pay" for the crime of slavery), to claim that their incessant America-bashing is actually a kind of super-duper ultra-patriotism.
All I have said is that slavery in America was a mistake and people believe I am a liberal for saying that slavery was a mistake.
With that, I can only assume that those who disagree with me do not believe that slavery was a mistake.