I seriously doubt that homebrew legislation is on BMC's radar (and really, we should be saying AB here, because they're the real big guns when it comes to lobbying). I imagine that if you asked them, their position would be "opposed", and since they do spend a lot on lobbying -- maintaining the three-tier system is crucial to AB's strangehold on the industry -- it's possible there is some incidental influence here, but I can't imagine it's a major factor.
OTOH, I also think it's naive to chalk it up solely to legislative ignorance. I have no doubt that the legislators are ignorant on this point, but I think it has more to do with the fact that it is tricky for an elected representative to take a position that can be perceived in any way, shape, or form as pro-alcohol, pro-drug, pro-crime, etc. Anti-drinking groups and conservative religious groups will scream bloody murder, regardless of how sensible the position may be, and it opens them up to attack ads in future elections, e.g. "Bob Smith voted in favor of legalizing the illicit production of alcohol in people's basements and closets, which would have opened up the door to rampant underage drinking. Why does Bob Smith think elementary school kids should get wasted?" etc.
Look at how hard it has been to modify the blue laws in NYS? Until a few years ago, you couldn't buy liquor on Sunday, and in my county you couldn't even buy beer until noon on Sunday. This was New York, for cryin' out loud! (Liquor stores still have more severely restricted hours on Sundays) In this case, there was some unusual opposition that came from small liquor store operators for whom Sunday was their day off, who feared they would be driven out of business by bigger stores capable of employing more staff (and indeed, I have seen one or two stores go out of business since then, but them's the breaks... if your business can only exist because your competitors are being forced to comply with a bizarre antiquated law that fringes on being a First Amendment violation, I dunno what to tell you...). But a lot of the opposition was also from anti-drinking groups, etc.
If there had not been an anti-homebrewing law to begin with in Alabama, I'm sure none of these politicians would care. But they can't be seen to be "pro-drinking", so they have to oppose legalizing it. I'm sure ignorance plays a big role too, but the imperative of being re-elected by an ignorant and judgmental electorate tells them what their default position should be for lack of any, you know, actual knowledge of the issue.