Soviet KMS Prime Mover and Firing....

[Poor Yorek]

5.6 Once loaded, the transport unit and everything it carries count as one unit for stacking, movement and combat purposes.

So the transport and the weapon are both carrying out the move action of "limbering/unlimbering."  Being free to move (hexes) is a continuation of the declared move action; fire is a different class of action.

Why? 

The only rationale I hear as yet is "because that's the way it was written"....I have yet to hear anything which addresses the
issues of "you can drive away at full speed, but you can't shoot"....

The answer to this type of response is simple.  You want to home rule something different, you are free to do so.  You want to play with someone else and they agree to those home rules, fine.  You want to play with someone else with RAW, then the rules are what they are (with all of their foibles).  

You do realize that this is a game, do you not, and that some compromises for rule integrity and simplicity might be necessary in lieu of "realism"?  If we started listing every deviation from reality in a PG ruleset, this thread would run for quite some time and PG would devolve into "Squad Leader for Tanks."