...I have always felt if they charged a miniscule sign up fee like say $10 they would get rid of 99% of the scammers.
Maybe but in my experience the pay sites are no better. They might not have quite the same volume but the ones they do have tend to be harder to spot, so waste a great deal more time.
And, by the way, $10 is not minuscule to some on this international site. In some countries, $10 is enough to feed and house someone for a day.
Overall, I am very happy that people run this service for free. It is incredibly good of them to risk their money and put in so much time for us. The scams come with the territory and can be found in any corner of the internet.