A couple of thoughts in response to some of the comments above:
1. hydrae states: "If they publish, that would be a report about an engineering subject, which would require a stamp".... I don't agree with you here hydrae - Simply publishing a paper on research in a professional publication is not necessarily the practice of engineering that regional authorities, or states, license. There are all sorts of articles written by non-PE's that are the result of expertise, lab tests, or basic knowledge that don't constitute the "selling" of professional services and don't affect the public safety and heath.
2. I think there is confusion here between the ability of a prof with a Ph.D. to teach and the ability of the licensed engineer to practice engineering. Each are doing separate things....although they revolve around the same subjects.
3. If a professor does direct research, though, then this is essentially selling engineering services to a client. My own major professor, for example, does research with composite metal decks for SDI and other deck mfrs. His tests result in written papers, reports, theses, and articles in professional journals that eventually work there way through the process and into a building code. This type of work I would think could be considered "the practice of engineering" and would require some type of licensure. Basic teaching of students would not. In fact, I would argue that the ability to know and teach engineering principles to students could not be measured properly by the PE licensure process anyway.