From the original Marathon Trilogy Box Set License:

(c) 1997 Bungie Software Products Corporation
All Rights Reserved. PO Box 7877, Chicago, IL 60680-7877
Marathon is a trademark of Bungie Software Products Corporation
All other trademarks are the properties of their respective owners.

From http://trilogyrelease.bungie.org/faq.html

Q: Hey, I thought Marathon needed serial numbers!
A: The versions released with the Trilogy Box Set were stripped of the serial number code. These will work fine - as long as you're not trying to play with folks who are using older versions.

Q: Wow... can I do whatever I want with this stuff?
A: NO. Bungie still holds the copyrights to these files. They're allowing them to be distributed for free (mostly because you can't buy them any more) - but they're still Bungie's intellectual property. You can't, for example, sell them.