This really sucks - just bought a copy of his MacOS app through his website to try and help compensate. At this point I'm starting to avoid buying software through the Mac App Store unless it's not available anywhere else. Even if it's slightly easier to make a purchase initially, you risk the headache of situations like this where you can't even migrate your license.
Additionally the apps outside the app store tend to work a lot better (nearly all the apps I wrote work far better in the website versions, not having the restrictions of the MAS versions), and also the developer gets more of the money, 95% instead of 70%. And then you're a customer of the developer, not of the app store, which is also better.
