[lmm]

But the physics of that is entirely wrong. See Olbers' paradox. Eyeballing it is fine; if you miss you will almost certainly never hit anything - it was considered fine to eyeball AA in WWII, and the odds of hitting a friendly fighter there were worse; also there are already plenty of big rocks going fast in space.

[TeMPOraL]

If you want to be charitable towards it you can assume it was meant to scare those recruits into submissions so they won't waste precious ammo in deep space battles. Also, the explanation holds in orbital fights when you're shooting downwards (wrt. closest planetary body).