Why is there a charge to e-file state?

I found this statement in a reply of another inquiry when I was searching for the answer to my question..."There is an extra fee to e-file the state.  You can avoid the state e-file fee if you print and mail the state instead of e-filing it."  Why is there a fee to e-file state taxes using Turbotax?  I do not understand why I am required to pay a fee to e-file?