Comment by tremon

But why are many Windows programs under C:\Windows\System32 then, if Windows has only a single model? Why aren't all Steam-provided (for example) games in a single location? Or, if they are, does Windows really have a single model?

Yes, the Linux/POSIX model is confusing, but the split is to segregate administrative domains:

- / and /usr are the domain of the distribution. As a user, you should never install there. The administrative group is root.

- /usr/local is the domain of the machine admin. If the machine is yours to manage, you can install software there. The administrative group is staff.

- /opt/$vendor is the domain of third-party vendors. Each vendor (like Steam, Eclipse, Arduino Studio) can get its own subdirectory and its own administrative user group.

How would you achieve the same on Windows? How do you make sure the Adobe updater can only install new versions of CS, but not surreptitiously install a new (free!) spyware package under C:\Windows? How would you allow certain power users to share one Google Chrome installation, allow each of them to update it, but not let them install additional software system-wide?