This product is either supposed to be a loss leader that brings more customers into the 37Signals fold (to use Basecamp, etc) or there are more features and functionality planned for the Breeze product.
My guess it's the later and its why Breeze wasn't just incorporated into Basecamp.
I could see people saying to themselves, "I love this email list thing, but it would be cool if we also had a calendar or could create lists, etc..." which prompts the upgrade to Basecamp.
The $10 one-time fee then becomes a way to separate the customers who are at least willing to take out their credit card for something like this, vs. those who would never do that. It will probably end up being a decent marketing tool that pays for itself over time.