To allow the person(s) who implemented it to get paid for their efforts and hard work?
It's also a very competitive price. $29 is excellent for a piece of software that helps my day to day. It's really a sweet spot between very reasonable, and a bit pricey. I'm so happy about this project, and hope it works well (I'll be trying it tonight after work)
No craftsman of any kind would whine about a (good) tool that costs $29 and makes him more productive. I do not see why a progrommer should do that.
On that same note, I wish we were more willing to pay for our tools as a community. If we were, I think more neat and productive projects like this might exist. Yet, developers seem to be historically cheap, and our love for open source (which I do love) seems to be mixed in with our willingness to spend money on our tooling.