Because the rulz we have all agreed to play by say it is so.
It's a game, not reality.
Sure its a game not reality. That's a valid point and true. But it's supposed to be based at least loosely on reality. If I wanted to play a game based on fantasy I would be playing Dungeons and Dragons.
One changes one's CA in this manner because one is fixing to fire elsewhere. If you're BU in an OT machine, by definition you're not fixing to fire anywhere.
(At least that's my rationale for it.)
I am not sure how I see this is really any different between a CE & a BU vehicle here. Neither is firing right now, just pointing the AFV in a general direction. If you're BU in an OT machine, by definition you're not allowed to fire anywhere. I am not sure that not being allowed to fire anywhere is exactly the same as not fixing to fire somewhere. I think the BU guy IS fixing to fire somewhere and will be ready to fire when he goes CE in the APh.
The best attempt at a reality rationale I've heard to date (this came up for me a couple months ago) is that changing TCA/VCA isn't just the AFV rotating by hitting the left/right pedal but that the AFV is moving from one firing position to another and w/o the Gunner be able to look around, that's kinda hard to do. That is to say, per your statement: " if they are not targeting anything but only pointing the vehicle in a general direction. " they are targeting potential threats coming from a certain direction. All the mini trees and shrubs and boulders and whatnot that would be in your way to shoot "that way" aren't on the map, after all.
You may or may not find that holds water for you.
So a smidge of designer intent and a smidge of possible unintended side-effect, that's my take.
Thanks but it doesn't really hold water, for me anyway. The vehicle is pointing his front in a general direction. One of 6 - 60 degree points around them where they think a threat may be coming from. I doubt that requires a lot of accuracy. I think any fine tuning of the gun to clear these vision hinderances you are speaking about would be done when the gunner was CE and actually looking for a target.
I mean no offence, but all the comments above sound to me like an excuse for a design oversight. That is OK. I guess the conclusion is that I accept that that is the way it is and will play the game by the rules as written. I don't dislike this fact enough to design my own game.
Anyway my apologies if my question upset anyone. I know how much everyone hates reality arguments around here and I wasn't really trying to start one. I was just curious if there was some rationale behind it I hadn't thought of, because it seems odd to me. So of course I wondered if the designers had consciously decided this and if so why? Sometimes it is interesting to hear why rules were decided the way that they were...especially if it is for some reason that had not occurred to you.
Finally I would like to thank everyone who replied. Hopefully we can all just agree to disagree on how we see this, and all stay friends.