Why is it that in most restaurants, you can drink many refills of coffee all for one price, but for tea, there are no refills (when it's really just a 3 cent tea bag and water)?

I was reminded of this issue when out with some coworkers earlier this evening.

I had coffee and got about three refills, and so did another woman I was with. A third friend had tea, and when she asked for more, the waiter told her that for another tea bag, she would have to pay for another cup of tea, but he could bring her more hot water and she could re-use the other tea bag.

This is the case in almost every restaurant I have been in. You can drink gallons of coffee (which is fairly expensive) if you want to, for the same price. However, to get more tea, you have to pay another $2 or whatever the charge is, just for a regular tea bag (which costs about 3 cents if you get a box of 100 for $3, retail price, which is more than restaurants would be paying). I understand that gourmet tea is more money.

Why is it that in general, in restaurants, you can drink a gallon of coffee for one price, if you wanted, but a tea bag is a few dollars each?