Why does the professor warn the student about a Web site he intended to use?

Listen to a conversation between a student (S) and an engineering professor (P).
S: You wanted to see me, Professor White?
P: Hi, Paul. Yes, thanks for coming in. l was looking over the bibliography for your paper. I’m afraid that some of your sources aren’t very reliable.
S: Oh, really? Which ones?
P: Well, one of your sources is John Anderson’s ’Guide to Renewable Energy Sources’; did you notice that this guide appears on a website that sells solar panels? You can’t rely on Web sites that have something to gain by providing information that favors their product. There are other more academic, and maybe more reliable websites dealing with types of alternative energy technologies, such as those associated with universities and scientific institutes. I know it’s not always easy to figure out which websites have trustworthy content, but...
S:OK. I see what you mean. I’ll go over my bibliography again and take out the sources I have that are like that. Since I am here, can I ask you a question about one of the technologies I am writing about?
P: Sure!
S: So, tidal turbines. Is it true that they are basically wind turbines? Though, obviously instead of being wind powered, water currents spin the blades of the turbines to generate electricity. Right?
P: Well, there are several kinds of tidal turbines. The simplest kind is called a horizontal access turbine, and yes with those the concept behind how they work is similar to how wind turbines work.
S: And, is it true that they are less damaging to the environment than other tidal turbines? I was wandering about that. Couldn’t they be more dangerous to the fish and marine mammals?
P: That is a good question, but these technologies are new and aren’t yet wildly used, so it’ll take a while before we understand their full environmental impact.
S: Ok. Actually, I am really interested in that aspect, the environmental impact of renewable energy technologies.
P: That is an interesting choice. A lot of people think of renewable energy sources as automatically good for the environment, but technically speaking any sort of development is gonna have an environmental footprint.
S: Well, I think this stuff with tides and water currents is particularly interesting.
P: If that is the case, you could do a more in-depth paper. One that focuses on what we do know, there are some tidal turbines in operation near the coast of Norway and one in operation near New York City. Look for some information on the Norwegian tidal turbines and how those have affected marine life.
S: Thanks for the great idea!