How upset would you be to pay $1.99 on App Store and later learn it's freeware! Would feel cheated and deceived and ripped off!
....and using the App Store costs money...
No charge for free apps
No credit card fees
No hosting fees
No marketing fees
https://developer.apple.com/programs/mac/distribution.html
Asking for donation is fine. Charging for your work is fine. However, IMO, they should not be charging for open source application on MAS especially since there is no cost if the app is free (on MAS). I understand developers need to get something back for their work and the price is minimal...but it is misleading practice.
Aren't there costs for developing or being a developer developing for the MAS? I am currently unsure and too tired to work it out (or too lazy to actually search myself)?