There’s a distinction in the US between fully-funded PhD graduate students (who receive a stipend contingent upon research/TA work in an employment relationship) and professional/masters students who pay tuition, even though the latter group is also referred to as graduate students.
Note that for instance if you're there with a fellowship and aren't getting a partial RA/TA assignment as well then you're not eligible. I know that some masters students pay for their tuition, so they might not be eligible either since they're not getting a stipend.