Sorry but I entirely disagree with you. From a security perspective allowing software installations form unknown sources is absolutely a concern for average users.

It's hard enough for IT professionals to maintain the security of general purpose computing devices, let alone non-technical users achieving the same goal.

Lets take a real-world exmaple. Look at the incidence of malware on a platform that allows unrestricted software installation (e.g. Android), with one that doesn't (e.g. iOS).

The problem is that making informed choices about whether a given piece of software is "trustworthy" is extremely difficult. Delegating that to a 3rd party (e.g. Apple) is for most people, likely the best option.