I don't really see much difference between "give us $99" and "we will only consider people who pay us $99 a month".
Edit: Here's what Jason has to say about it in the comments:
It’s no different than entering a design in a design annual or contest. You pay to be included, they publish a book, you get exposure. Thousands and thousands of design firms do this every year. They pay to be included in a promotional piece in the hopes that someone will see their work and consider them for a project.
I also liked the person that pointed out that only 86 people have to sign up or upgrade for 37signals to get their redesign for free.