For free apps you don't necessarily need to enter a password, for paid apps it's still simpler than whatever payment system the developer might be using.
With the MAS you can go from viewing the app/developer website to the app being installed in as little as two clicks.
How is that worse ux than manually downloading an archive/image, opening the file, moving the .app bundle or running the installer.
There are some valid criticisms of the MAS. UX on the install process is not one of them.