Connect your phone to your laptop, go to settings->More...->Tethering & mobile hotspot->USB Tethering. When you select this for the first time, your phone installs a couple of extra drivers and you see a new Local Area Network connection (remote NDIS based internet sharing). Now, this option is for sharing your Mobile internet with your laptop.
What you have mentioned in your post is Reverse Tethering (sharing your laptop's internet with your phone via usb). So I looked into it and this is what I found out.
I enabled USB tethering on my phone and then I tried sharing my laptop's Wifi (On your laptop go to Control Panel\Network and Internet\Network and Sharing Center\Change Adapter Settings->Right click the Adapter (WiFi or Ethernet or USB dongle) you want to share to your Tethered "Local Area Network"->Sharing Tab->Tick all those boxes and Choose "Local Area Network" from the drop down list. Don't confuse this with your Ethernet (look for remote NDIS... instead). So I finally came to the conclusion that internet does not enable on your phone. It seems that Motorola has not included this feature unlike HTC and other manufacturers, as others have pointed out here.
Rooting or modding your phone will ultimately void your warranty and discussions about the same are not allowed on the forum.
Try installing Connectify on your laptop. There's a trial version available I think. It turns your laptop into a WiFi hotspot. And to be frank, its more convenient than a USB cable.
