In this case they filtered for Americans. They put some effort into validating identity, and into ensuring that you are taking the survey seriously (trap questions, for example).
It doesn't strike me as sufficient but it's not my field.
I can say (but not visibly prove, unfortunately, NDAs) that large mechanical Turk surveys tend to get distinctly different results to mail and phone call surveys. Enough so that the commissioners of the study were not interested in replacing existing surveys with mechanical Turk.
It doesn't strike me as sufficient but it's not my field.