That's been generally how things are on OS X for years (even before the Mac App Store). Small companies like Panic (much smaller originally) have found out that people are willing to pay for quality tools.
Except the Markdown editors phenomenon is slightly different. The tools Panic makes are complex and useful; but there seem to be a huge number of hugely similar "editors" for a format which was designed to be written in plain text. Typically these editors don't add much tangible value over a general purpose text editor and cost twice the price.