Plus the fighter can get in close and use its factor 6 m-drive as an energy weapon...
With a comp m/1 and a DM -6 to penetrate, they will still have a hard time hitting anything.
But, agreed: a few thousand fighters should have little problem scraping the weapons off a battleship at a fraction of the cost, unless the BB has enough Sand to make penetration impossible.
As the BB can only attack a few fighters, the fighters are practically immune to counter-fire.
Effectively a BB needs a factor 9 sandcaster (9 triple turrets per thousand Dton), and hence very limited offensive weapons, to be survivable against light fighters.
To make the fighters capable of penetrating sand-9, they would only have to have a m/3 computer, adding a few Dton and at least 100% cost.
A thousand unarmed comp m/3 fighters would produce ~8 damage rolls per attack, so roughly 10 weapon hits and 8 M-drive hits. So, three-four thousand fighters would mission-kill any possible ship in a single attack (or much fewer if it doesn't have Sand-9). The ship can't fire back effectively, regardless of how many weapons it has.
Note: The armament of the fighter is completely irrelevant, as it is low-factor and can't penetrate without a much larger computer...
And again the conclusion is: HG'79 is a completely ridiculous system...