I agree - this matches my experience too. Americans are blunt but that goes both ways. They appreciate a good deed while at the same time make their disapproval heard too.
I would point out that "Americans" are not a homogenous entity, culturally or otherwise. The United States is huge and has many distinct local cultural regions, with radically different behavioral norms that are obscured through the use of a common language and political system.
San Francisco is vastly different than New Jersey, for example, and both are vastly different than, say, the rural south.
San Francisco is vastly different than New Jersey, for example, and both are vastly different than, say, the rural south.