Ron Gula weighed in on this question as posed to he nessus mailing list today. I'm trying to read between the lines here, but I think Ron's answer boils down to, "It pisses me off when people pretend they don't use Nessus and just re-style our reports to customers as original content. But since we don't own the IP in a good number of the NASL files including the report output that they generate, no, it's not illegal." I'm liberally paraphrasing, of course.
Food for thought, because it's almost standard operating procedure for pen-test companies big and small alike to not broadcast their use of Nessus. But next time you hire someone to do a pen-test of your network, grep your web server logs for Nessus - I'll bet it hits. As to how many copy & paste Nessus results directly, only the laziest do it straight up, since there are typos and layout mistakes galore.