They really are not charging sales tax. They are charging use tax. Sales tax is on tangible property. However, michigan and others have a use tax which is charged for services and tangible product purchased outside the state for use in Michigan. The rate is the same as sales tax. The correct sales tax terminology in Michigan law is Sales and Use TAx.
The only way you can know if they are paying, is to check with the Michigan Sales tax deivision of the treasurery and ask them.
You can also demand that you be provided a copy of the sales tax certificate for Virgin Mobile (demand it of Virgin Mobile, not the treasurey).