Sean is on the trail of something on MS-USERS ..
I couldn’t be bothered checking whether the software has the required MIT License enclosed, but lets hope they do! While nothing is hindering piggy-backing on OSS projects, it does certainly strike a choord as to doing the “right-thing”. DMSolutions commercial products certainly know that balance, lets hope they do the same.
Related posts:

That was an ancient thread revived by Bart, and I did some blogging about it at:
http://zcologia.com/news/348/beneficial-proprietary-extensions-of-open-source-software/
http://zcologia.com/news/370/proprietary-feedback-part-2/
I have confirmed they follow MapServer’s license and include the copyright notice.
Damn that Bart! Interesting posts Sean, cheers