Unfortunately I can attest personally to the fact that this is a misleading statement. YES, grad students are often paid a small salary. In the sciences it is usually for some type of a lab research study done on federal or private industry grant money. BUT, it does not cover much more than basic living expenses. It would normally be between $1500 and $2000 dollars a month at the most. Below is one ESTIMATE I found handy to gauge the cost of the average tuition and fees and living expenses for a graduate student in the USA.
Tuition - $43,930
Room & Board - $18,973*
Books & Supplies - $1,200
Personal Expenses - $1,300
Travel - $500
Mandatory Health Insurance - $1,800 (estimated)
Matriculation Fee (1st year students) - $500
Total Cost - $68,203
http://www.grad.jhu.edu/admissions/cost-financial-aid/
The starred amount for room and board is what is normally covered by being a "paid" position.