Needless to say, a paper purporting to show that demand curves slope upward to the right has been controversial.
Here’s are a few answers to Card and Krueger:
http://mises.org/daily/2596/Minimum-Wage-Laws-Economics-versus-Ideology
http://mises.org/daily/6638/Welfare-Minimum-Wages-and-Unemployment
https://mises.org/daily/2130
Here’s a piece claiming that Krueger himself disavowed the conclusion you cite:
http://www.forbes.com/sites/susanadams/2011/08/31/obama-nominee-pulled-back-on-minimum-wage-defense/
