I think you could set it up legally like this example without violating anything and depending on how your server is set up you could allow different owners of the site1 site2 and www subdomains.
main site www.example.com (which is licensed and owns the forums)
forums www.example.com/forums/
site 1 site1.example.com (with link to forums)
site 2 site2.example.com (with link to forums)
etc,