TimeWarner Cable automatically assumes you’d like to lease your cable modem with them. You don’t have to. There is no law that says you have to lease their modem. 
Cable companies charge at least $3 per month when leasing their modem. You can buy a modem for $40-50 and save on the year 2 and beyond with your cable modem bill. There is actually a good sale available on D-Link modems that is $19.99 after a $20 mail-in rebate.
Make sure your cable company is compatible with the D-Link modem although they are generally supported. It is wise to call up your cable company first to see what they are charging you monthly and then you can make your decision. Why not save a few bucks?