Fundamentally I think it comes down to a choice made by the core developers and nothing more than that. Why? Because by default, there should be almost nothing of any value to anyone in /root. No one should be logging in as the root user for general stuff.

For example, on FreeBSD everyone can read /root. Some files within /root can not be read for security reasons but you can still "see" those files are there with ls (just can not read them). For example, .history is set -rw------- but .login is -rw-r--r--.

FreeBSD has a slightly different approach to security to Linux. Historically FreeBSD has been for servers and while it can be run as a Desktop it really is better (by default) as a server.

Personally, I see nothing wrong with this set up (/root can be read).

The /root on FreeBSD has almost nothing in it except for configs really. Mail should be forwarded to a real user. No one should be logging in as the root user. The account should only be used for installation of and configuration of software as well as maintenance tasks. Other than a few security sensitive files (like .history) there is nothing to hide in /root in my opinion, on FreeBSD.

For more reading on this, try the FreeBSD handbook section on security. I did not see anything on their choice to make /root readable in a quick scan but there is a lot info there.