I know of companies that do this, and I totally agree with you. It's done under the guise of security, but is really just immature hazing. First, if I'm in an office environment with fellow employees why is locking my computer so important? Is it to stop the random person from stealing company secrets or impersonating me? If that's the case why wouldn't the other employees around my computer notice someone, since clearly they had to be close enough to notice if I left it unlocked.
Second, if I'm out in public theft is much bigger issue than locking the computer.
Finally, and something you touched on, is that now it makes it hard to differentiate between authorized and unauthorized. It's much easier to say using someone else's computer without their knowledge is grounds for termination 100% of the time.