Actually, they make a lot more money on "breakage". This is the rebate refunds that "break" - i.e., the customer doesn't send the rebate form back for whatever reason, it's not filled out properly, isn't mailed before the cutoff date, etc...
The offline stores like CompUSA, Staples, etc., who also do free items after rebates, do so not only as loss leaders to attract traffic to their stores, but they often actually make some money on those items.
Every loyalty point system out there heavily depends on "breakage" to increase profits. Also, pre-paid calling cards use breakage to get healthy margins - some people lose their cards, others don't use 100% of the card, etc...