answer:
I haven't really come up with an answer to this question, but to share a few thoughts:
I'm not sure that it's possible to find anything that says that discrimination is "allowed," but perhaps it might be possible to find arguments that correlate "right to work" laws with discrimination.
Here are a few starting points:
-Raymond L. Hogler, The Historical Misconception of the Right to Work Laws in the United States: Senator Robert Wagner, Legal Policy, and the Decline of American Unions, 23 Hofstra Lab. & Emp. L. J. 101, 105 (2005–2006). PDF
-AFL-CIO website: "Right to Work for Less"
