The app store mentality has made people forget that some software isn't the price of a RedBull energy drink. I was paying $30-$40 dollars for games for my C-64 back in the 1980's. If $20 is too much for someone, there are much cheaper alternatives..
I just bought it because of the features. I use Tweetbot on my iPhone and iPad. The iCloud synching of mute filters and tweet position make it worth the money to me. Sure, I personally expected it to be cheaper, but if Twitter is going to ration the number of available tokens, therefeore limiting the number of allowed users, then it makes business sense for TapBots to price the software accordingly. They can only sell X number of copies, and they need to re-coup the development cost and make a profit. Thats business!
My main reason for choosing it over others, such as the very good Osfoora, is keyword muting. Now my twitter stream is almost completely "political BS" free 🙂