I think the truth lies in the middle here.

People that make movies / pictures own the copyright to that material.
They have the right to ask you money if you want what they own.

But on the other hand, what is the use of making such movies if nobody is going to watch them, because it costs too much ? Especially adult movies that target a small group of possible consumers.

Of course most of these movies are made by amateurs, because it is hard to make a professional living out of it. They should get some money, because it is much more fun if you make a little money out of your hobby.

So to the extremes: If they would make loads of money, production of these movies would not explosively grow.
And if they were unable to earn any money, people will not stop making such movies, just because people like doing it / watching it.

A little money is needed to keep people a little more intrested than just hobby-wise, and to keep up a little quality, so that you dont watch worthless crap, shots made with crappy camera's.

I think people who sell these clips keep in mind that maybe 2% of the actual viewers buy it legally at their website. That is why they ask these high prices. In theory if they would lower the prices to 10%, and all viewers would buy it legally they would make more profit. But the fact is, most people who watch these clips will never pay for them. It is not just the amount of money that makes people decide to buy or not, it is also the step of actually buying it.

In stead of asking people not to spread these clips, you should ask people to please buy a clip every now and then. It is only fair.