Yes, it's good that if you sign out of the App Store, you can't accidentally purchase something as you browse. What about those situations, where you are browsing, find an app you want to buy, and so you sign in, download it, and then accidentally click as you move the mouse away, thus purchasing, without any warnings or notifications or password prompts, another app?
Look, there's something to be said for being careful and responsible. However, making it so that a single click, without any chance to stop the process, could potentially cost you hundreds of dollars that you then have to spend the next few days trying to recover, is not good business for Apple. They don't want that, because doing refunds will be an added cost. I will guarantee that they will quickly issue a fix for this, and turn off the one-click buying unless the user opts in.
Is it the worst thing they could ever have done? No. Does it mean the Mac App Store is evil and should be avoided? No. Have I accidentally purchased something on the Mac App Store because of it? No. Do I think it should be fixed?
Yep.
And you can bet that plenty of other people want it fixed too, and among them are some of the people at Apple who will be involved in the decision. I can't see this functionality not getting improved, and quickly.
jW