The problem is that when one mounts a Pentax lens without any "A" position, actually it does not work "perfectly". It cannot do metering by default, what the user must do stopped down metering by pressing the green button (for *ist D firmware 1.1 or later) and the AEL button (for *ist DS/DL/DS2) before releasing shutter. Otherwise, metering will not be performed (and is impossible/unable to be performed too, owing to the cripped K mount used in the *ist D series DSLR bodies) and incorrect exposure will be resulted.
To place safe, Pentax opted to lock the operation of the camera by default, if the camera cannot see the "A" on lens. The good thing is Pentax still let the users to use the non-A lenses only if they know what to do and why is that.
The last thing is that the stopped down metering is not as accurate as the wide opened metering on the *ist D bodies afterall, too.