First off, in terms of whether independent contracts can be asked different questions, all I can find are that it's legal to ask specific questions if they directly relate to the ability of the applicant to do the job. The Department of the Interior has a useful table of legal / illegal questions.

After digging around quite a bit further, the best answer I've found was on the terrific Business.Gov site. Here's what they say about this very important subject:

The Successful Interview - What Not to Do

The following list is comprised of subject matter that is widely regarded as off-limits for discussion in an interview by employment experts. Most of these subjects relate directly to federal and state employment laws. Legislation covering equal employment opportunity is extensive and complex. Check not only federal laws, but also your own state's laws and guidelines. Remember, state laws vary! Consult an attorney for legal advice before you begin the search process for a new employee.

In an interview, or on an employment application, do not ask questions:

• Concerning the age of the candidate. Be careful using the words over qualified with older candidates.
  
• About their arrest record (this is different from convictions - in most states, it is permissible to ask if the candidate has ever been convicted of a crime).
  
• About race or ethnicity.
  
• Concerning the candidate's citizenship of the U.S. prior to hiring (It is permissible to ask "Will you be able to provide proof of eligibility to work in the U.S. if hired?")
  
• Concerning the candidate's ancestry, birthplace, or native language (it is permissible to ask about their ability to speak English or a foreign language if required for the job).
  
• About religion or religious customs or holidays.
  
• Concerning the candidate's height and weight if it does not affect their ability to perform the job.
  
• Concerning the names and addresses of relatives (only those relatives employed by the organization are permitted).
  
• About whether or not the candidate owns or rents his/her home and who lives with them. (Asking for their address for future contact is acceptable.)
  
• Concerning the candidate's credit history or financial situation. In some cases, credit history may be considered job-related, but proceed with extreme caution.
  
• Concerning education or training that is not required to perform the job.
  
• Concerning their sex or gender. Avoid any language or behavior that may be found inappropriate by the candidate. It's his/her standard of conduct that must be met.
  
• Concerning pregnancy or medical history. Attendance records at a previous employer may be discussed in most situations as long as you don't refer to illness or disability.
  
• Concerning the candidate's family or marital status or childcare arrangements (it is permissible to if the candidate will be able to work the required hours for the job).
  
• Concerning the candidate's membership in a non-professional organization or club that is not related to the job.
  
• Concerning physical or mental disabilities (asking whether the candidate can perform the essential job duties is permitted). The ADA allows you to ask the applicant to describe or demonstrate how they would perform an essential function(s) when certain specific conditions are met . Check the law or consult with an attorney before moving forward.

Remember - when in doubt, ask yourself if the question is job-related; if not, don't ask!

For more information

The U.S. Equal Employment Opportunity Commission homepage: http://www.eeoc.gov/.