is an open protocol meaning that anyone can use it. It is specifically designed by the lawmakers to allow third parties to maintain cars and not restrict repairs to authorized dealers.
http://tech.groups.yahoo.com/group/opendiag/ would give an insight to the many people using that interface.
Now somehow using an open protocol for what it is designed for as Durametric does seem ok to me. See Actron et al.
Porsche has lawyers and somehow I think that if Durametric was violating some right that Porsche wished to assert via a cease and desist, Durametric would have heard by now.
Now if Durametric specifically permits by license ( as opposed to merely feeling there is no point in incurring the legal costs of enforcing the license) the use of their s/w with third party cables, then I owe the OP an apology. And if his intent was to point out the advantages of buying the real Durametric cable and s/w then again, I apologize. There is no s/w license info on the Durametric web site that I know of. (Yes, I read the darn things before ordering. I wrote them in a prior life.)