Are test posting sites ethical?

As a college student I am always looking for study materials and better ways to learn what I need to know. I am always extremely excited when a professor posts previous tests for their students to study from. The newest craze in the college education world are the websites where people (professors, students, bookstores) can post prior exams for current students to study. Two of the most popular are koofers.com (a site named after the bins tests are kept in at universities) and PostYourTest.com.





The biggest question when it comes to these sites are if they are ethical or not. Is it okay to look at previous tests?
For students it feels like the most amazing thing ever. But are they even helpful if the professor changes the exams for every semester? I guess if it reinforces the subject they could be great practice sets. But is this process fair for those professors who do not change their tests every semester? Is this another way to cheat for those students looking for more than just another study tool?