Many different laws prevent discrimination in hiring. They limit potential employers from asking questions about things such as race, religion, age, and disabilities.
They do so because those items aren't relevant to whether one can do a job. Employers can ask questions to confirm that essential duties can be performed.
For example, we can ask if you are available to work on Saturday mornings. We can't, however, ask if you are Jewish and then make an assumption on how that limits your availability on the Sabbath.
At this point, however, it is common practice for employers to include the question, “In the last seven years have you been convicted of a crime other than a traffic violation?” on their job applications and make assumptions based on the answers.