They typically limit the number of hours you may take per semester. The most I've seen is 6 hours (about 2 classes) per semester.

NYU does this as well, either 50% or 100% tuition remission depending on the employee for up to 9 credits per semester. It also covers 50% of course registration fees.

When I worked for the University of Chicago, they offered to pay 50% of tuition as well. But 50% of tuition for full time enrollment was more than my salary there :)