There have been a few different conversations on this, especially, last year when we changed our license terms for moving forward. The summary really comes down to: the current license terms were put together in order to make self-hosting viable for the future for both our clients and us. The self-hosting market has shrunk to a very, very small part of our business (and respectively, the internet as a whole) so if we are going to offer a self-hosted option, we want it to be the best of our software suite and if we're going to sell it, we don't want to abandon our self-hosted customers so it has to be something we can continue to provide support to at a renewal which works.
It's also worth mentioning that you get what you pay for. Our software is ever evolving (check out our Version 5 Blogs). We offer monthly releases, support, Spam Defense, GEOIP, and a lot of other goodies, all for $499 one time price and $199/year afterwards.