Thanks for the additional feedback! Do keep in mind that this was simply one possible work-around for a bug in the NuGet client that they fixed more than a year ago. Also do note that, when you grant a user Feeds_AddPackage to a user, they only have access to do that task. Which in this case, is very minimal. They do not become ProGet administrators, etc.
In Windows, you can actually configure all users to be "Domain Administrators" (or have "root" open in Linux). We offer the same flexibility in ProGet, and you can configure ProGet to be as secure or open as needed. This is actually a very common feature in software/vendor products and, like most software vendors, we empower users to decide how secure or open to make their instance based on their specific needs and requirements.
Unfortunately we can't offer specific advice to everyone on our forums on how to decide which policies suit them, but we do have some training and certification courses which offer lots of security topics (including awareness of the "security hole" you described, as well as lots of other potential attack vectors).
You seem to be quite passionate about organizational security (i.e. InfoSec), which is awesome! There's a ton of great sources out there, but I would highly recommend you follow Krebs on Security and Schneier on Security. Those are some great starting points which you can use to starting learning how to develop best practices in your own organization :)