Laptop must use a router with Time Warner?

Just wanted comments on something I find quirky.

I bought a new Studio XPS 16. When I tried to hook it up by cable directly to my cable/phone modem from Time Warner (Arris TM502G), the way I used to do with my desktop, the internet connection was very weak and kept failing. I updated Vista, the driver for the Broadcom network (card?), disabled TCP/IP 6, reset the modem a few times, disabled the network power saving option, but to no avail. For instance, I could never perform a successful test on speedtest.net.

I went to a friends house and was able to use their wireless network (also Time Warner), internet worked, no problem. Then, I closed off the wireless connection and connected by cable directly to their router. Internet worked, no problem. I took that cable and went home and used it to hook up to my cable modem, same problems as before.

I called Time Warner, and the assistant (who did not sound by voice to be a technical person at all) told me that if I am using a laptop, I need a router and cannot connect a cable directly from the laptop to the cable modem. I told her "So you are saying that I can hook up a desktop directly to the cable modem but not a laptop." "Yes, thats right". I was shocked but said "OK, Ill try it and see what happens"

Sure enough, I set up a router, and now my internet connection is perfect, whether I used the wireless or connect the laptop directly to the router with a cable. So she appeared to be correct, yet it doesnt make any sense to me?!?

Has anyone ever heard of this? Is she in fact correct, or was there something else going on? If this is correct, what exactly is missing in a laptop that makes it require a router?