If you simply have the hub pluged dirrectly into the cable modem without first going through a NAT or a proxy server, then yes. They can see every computer and traffic to each port on each independent computer. Their server logs most likley have records on traffic acessable. If they see unusualy high traffic on your account, you may have some explaining to do..
I gurantee if you call them on this, you will loose your account. If you read your EULA, which is probably posted on their website, you will see that they highly frown upon this. While it isn't illegal, they consider it to be not playing fair and they will do everything inside the law to make your life miserable.
As I said before. As long as you dont do something stupid, then you are fine. There is little legal recourse they could take against you for internet connection sharing. I have been doing it for over a year now with out any problems. Get yourself a copy of sygate, load it on a machine with 2 NICs, plug everything up and you are ready to fly. If you are running a NAT server, ie sygate, the cable modem conpany will see nothing unusual in their server logs but odd port numbers. They will only see the gateway machine, the server, because it is the only one with an IP address that they can scan. The machines behind the server are completly invisible to the outside world.