It seems like the author of MacDown might also have some sort of a philosophical axe to grind with the author of Mou (http://mouapp.com), another popular (albeit non open source / future non-free) Markdown editor. Mou implies that it will become non-free once it leaves the beta, but it's been in beta for years now.
That doesn't seem to be a source of the issue though. Navigating on to the homepage of the application, the author (of MacDown) writes more about the reason there. Apparently, the author of Mou, wants to stop developing Mou and sell the project and the author of MacDown didn't have the cash and decided to write an open source clone of it instead.
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Why Another Markdown Editor?
I like Mou. I write Markdown all the time, and since I use OS X on a daily basis, Mou is my go-to editor whenever I wish to generate something with markup. But I had always wanted something more.
It came as a great shock when Chen Luo announced that he felt he could not actively continue the development, and wished to sell the ownership of Mou. No suitable offers surfaced (I honestly do not think there will be, either), and I decided that instead of waiting for others to do something about this, I should act myself.
I don’t have nearly enough money to match Chen Luo’s purposed offer, but I do have my own pocket of tricks and some free time. So I started from scratch, spent some weekends hacking together my own solution. And this is the result.
the Mou developer has declared that he no longer intends to work on it and attempted to sell it on a Chinese Ebay clone. I'm not sure if he was successful or not.
That being said, the plan was ALWAYS for it to be non-free when it left beta. It just never left beta. Now, it's either going to wither and die OR the new owner (if there is one) is going to want to recoup their money.
The source of the issue seems to be a tweet by Mou's author in which he says that when people say "inspired by X" they actually mean "idea stolen from X." (https://twitter.com/remaerd/status/484914820408279040).