Password protection should be the default for ... Single User mode
Consider what happens if the root password has been lost, or if the passwd.* file which stores the passwords has been damaged. Now you have a bricked machine. It can still be recovered (just take the root disk physically out, and move it as a non-root disk into a different machine), but that's much harder. In particular for a newbie (who doesn't know how to do this) or a household (which is less likely to have a spare computer or disk around), this is a particularly tall order.
I also object to it on philosophical grounds. It gives the user a false sense of security. They will think that even though an intruder has been able to get physical access to the console, the data on the machine is still safe, when in reality, it isn't (because the intruder can now physically pull the disk out and take it). Your talk about "difficult without being detected" is nonsense. It is perfectly easy to detect when someone becomes the super-user, or when someone reboots the machine (which is required to go into single user mode). And the most serious of the attacks you describe (become super-user, do "mount -a", then modify files) is perfectly easy to execute, if one carries a USB stick with a bootable OS around: just plug it in, hit the reset button, and go to town.
Why do you not think that this is a serious security breach?
It is a serious security breach. If you want to protect against it, do so. The correct way to protect against it is not to put a meaningless doorknob on single-user mode, but top protect the hardware. There are many ways to do that: physical security, encrypted disks, and so on. Most people chose to not protect against it; I think in most cases that is a sensible and rational choice.
I do agree that FreeBSD is more authentic to UNIX than Linux ...
The whole discussion of what you consider to be "Unix" is silly. There are many ways to define "Unix", for example: Posix compliance, contains Ken and Dennis' AT&T source code, or contains Kirk's Berkeley source code, and so on and so on. For most purposes, Linux is Unix (the by far most popular one), followed by the various *BSDs. Microsoft Windows and IBM's mainframe operating system (today I think called zOS, formerly known as MVS) both can claim to be Unix too, as they are Posix compliant. If you want to know what the "Unix philosophy" is, that term is more often abused than used.
One of these days, I'll ask Ken what computer he carries around with him, if I happen to see him; unfortunately, we can't ask Dennis any more.
I had vaguely heard of OpenBSD, but quite frankly I don't think that it should even exist. I don't comprehend any reason for there to be more than two UNIX-based freeware Operating Systems ...
There are lots of good reasons for OpenBSD to exist. It is a very fine operating system (even though I admit to not having used it in about 6 or 8 years now). It has its strengths, and also its weaknesses. So does FreeBSD, so does Linux, and many of its distributions.