Griggs v. Duke Power Co. established the prohibition of tests in hiring decisions where the test is not directly relevant to the work and has a disparate impact on protected groups. IQ not generally being directly relevant to any line of employment (despite being highly predictive of success), and skewing notably on racial lines, is typically considered off-limits. See also this article on Wikipedia.

Employers that strongly value intelligence in hiring decisions may make a deliberate effort to identify work-relevant or non-discriminatory tests that are highly g-loaded to circumvent these issues.