Yes, they are not pure volunteers. They do get paid and their incomes are taxable.
Salary and Benefits
MSF salaries are set so as to reflect the humanitarian spirit of volunteerism while recognizing the high level of professional expertise provided by field staff. Starting gross monthly salary is approximately $1,731, with subsequent increases based on expertise and experience. Americans working in the field with MSF receive a US letter agreement in which all required taxes and contributions are deducted.
See link.
http://www.doctorswithoutborders.org/work-us/work-field/benefits-opportunities