Why is employment by a labor union considered self-employment?

It's not clear.  I would think you should be an employee, but you can use the following form to decide. The IRS uses it to make determination as to whether someone is an employee or if they are considered an independent contractor/self employed.

The Form SS-8 helps to differentiate between an employee and an independent contractor (self-employed). Take the time to review this form and decide if you have an employer/employee relationship.  If the person in charge tells you when to be there, how to do your work and provides all the equipment necessary to do your work then it's quite likely this is an employer.

• Instructions for Form SS-8
• What is Form 8919? (To be used if you want to file as an employee, you will need all of your income information) 

You are allowed to report the income as an employee if you believe that is the relationship you have, however, this will not go well with the labor union.  The IRS will go after them for payroll taxes which could mean penalties.

You can also continue to report your earnings as a self-employed independent contractor.  To file like this you will need to use TurboTax Online Self-employed or TurboTax CD/Download Home & Business.

• Where do I report Schedule C income?