I just got off the phone with customer service and was told that note should not have been on my bill. She told me the $5 modem fee is only for those not already paying $5 for the HSI modem.
Base on what MediacomChad said and the verbiage on my bill I'd guess she doesn't know what she is talking about. She assured me that she is 100% correct.
"Mediacom works hard to control our costs in order to keep your prices fair and reasonable. In spite of our efforts, increased expenses now make it necessary for us to change our equipment rental fees. Effective with your next statement, a $5.00 phone modem rental fee will apply for Mediacom Phone customers. This fee is in addition to equipment fees that apply for Internet service. Customers who are in a promotion or a contract will not see these changes until their promotional period ends or their contract expires. For customers who do not wish to rent, Mediacom offers modems for purchase. Call us for details."