Well. They allow you by not stopping you, directly. But I know think that their license doesn't allow it...
LICENSE TERMS
Following acceptance of this Agreement, you may install and use one (1)registered copy of the Software for your own personal, individual use,subject to the terms and conditions set forth in this Agreement.
Use on a Personal Computer.
1. You may:
a. Install and use a single registered copy of the Software on one harddisk or other storage device for one personal computer, and you may accessand use the Software at that location so long as only one copy of theSoftware is in operation..
b. Install and use a second copy of the Software for your exclusive use,and as the primary user, on a backup or portable computer, so long as eachregistered copy of the Software is used on only one computer at a time.However, you may only install a second copy of the Software on a second computer only if you are the main and primary user of this computer (e.g.,a second home computer or laptop) and the Software installed on the secondcomputer is linked to your Impulse account.
c. Install and use a copy of the Software on a new primary computer orreplacement primary computer only if you change or replace your primarypersonal computer and uninstall the Software on your old primary computer.
d. Install and use the Software on a network only if you have legallyacquired a separate license for each workstation on the network. The numberof users on the network must not exceed the number of licenses for theSoftware you have legally acquired.
e. Make a single backup copy of the Software for archival purposes only.
So it's legal for me to keep one copy on my notebook, and one on my desktop and switch back and forth between each other. It's illegal for me to let my friend play on my notebook at I play on my desktop on a LAN with him. Similarly, it's illegal for me to copy it onto my roommate's computer so that we can LAN together.