Well, I'm just a user, just like you, so I can't answer why questions in terms of developer motivations.
But I can tell it it doesn't work that way. MC makes some attempts to correct for user actions like yours, but they're only effective up to a point.
I can also tell you that MC is a database. If you understood more about databases, you would realize that using the OS to move around files that are referenced in a database, without going through the proper procedures to make the database software aware of those changes, will cause problems. It's like that in all databases, big or small, for several technical reasons. So yes, your expectation is unreasonable. But it's ok, a lot of people go through the same thing. There's a learning curve.
So instead of just "trying again" in terms of doing the same thing, I suggest you use the RMCF tool next time, as I mentioned.
You should base your procedure around the following:
1. Put the drive letter back the way it was.
2. Restore the library from before your actions.
3. Set Fix Broken Links=NO, Disable auto import, and make sure it is not configured to run in the background.
4. Make another library backup. Observe the all is as it should be in MC.
5. Now you have two options:
a)either use RMFC to move the files (if you can have both drive letters active at once)
or b) change the drive letter, do whatever else you want to do to move and rearrange your files, and then use RMCF with the "Update database to point to new location" option selected in the dropdown.
Again, read the wiki article you find by googling "jriver moving files" to learn how to use the tool.
After you're all done with your moves, you can re-enable and reconfigure auto-import if you want. Using Auto Import (especially allowing it to run in the background) should only be used if you know exactly what you are doing. I recommend Fix Broken Links always be set to NO.
Good luck.