See this part of their agreement where it says yo can Buy your own Cable Modem or Router:
1. Equipment; Access.
1. Required Equipment: You understand and agree that the Service requires certain equipment provided by you, including but not limited to, a personal computer, an Ethernet or other networking device, if required, and an appropriate operating system (the "Customer Equipment"), as well as certain other equipment installed and/or provided by Comcast or its designee, including but not limited to, software, hardware, and external wiring owned by Comcast ("Comcast Equipment"). If you are leasing a cable modem, wireless gateway/router, or other device from us, the cable modem, gateway/router, or other device is Comcast Equipment. If you have purchased a cable modem, gateway/router, or other device (whether from us or from a third party) the cable modem, gateway/router, or other device is Customer Equipment. If you purchase your own cable modem, it must meet the DOCSIS compatibility requirements established by CableLabs, Comcast, and its underlying providers. If you purchase your own gateway/router, it must be a CableHomeTM-certified device in order to receive customer support for it from Comcast. Comcast reserves the right to provide the Service only to the extent that you use equipment, including but not limited to a cable modem or gateway/router, that is compatible with the Service and meets Comcast's specifications and requirements. If you use any equipment that is not compatible with the Service or does not meet Comcast's specifications and requirements, you may not be able to use the Service or some features of the Service and we reserve the right to terminate the Service. Whether a cable modem, gateway/router, or other device is owned by you or us, we have the unrestricted right, but not the obligation, to upgrade or change the firmware in the cable modem, gateway/router, or other device at any time that we, in our sole discretion, determine is necessary or desirable in order to provide the Service to you in accordance with our specifications and requirements. We may perform these upgrades or changes remotely or on the Premises as we determine in our sole discretion. In certain cases, you may need additional specific equipment in order to receive certain Service features and/or functions. All of this equipment must also meet Comcast's specifications and requirements.
But then a bit further down, they tell you you can't hook up more computers than you signed up for or told them about (condradictory) Section G
Theft of Service: You will not connect the Service or any Comcast Equipment to more computers, either on or outside of the Premises, than are reflected in your account with us. You acknowledge that any unauthorized receipt of the Service constitutes theft of service, which is a violation of federal law and can result in both civil and criminal penalties. In addition, if the violations are willful and for commercial advantage or private financial gain, the penalties may be increased.