They are just a business trying to make a buck, and sadly, that is what makes them somewhat unique. Twitter, Facebook, YouTube are "successful" and get a ton of press, but they've yet to make a profit. 37signals isn't afraid to charge for a product they know is great.
37signals is really good at making you realize how these web companies might not be the best business to get into. Could there be a bubble slowing building that will eventually burst?