I'm not an expert on this subject and as far as I know none of our Black Bay owners have had an issue with it (although we haven't been selling them that long, in fairness). That disclaimer aside, from studying various photos, it appears to me that the aluminum tube is decorative, which makes sense since you'd probably want to use a harder material for this sort of screw thread. At any rate, whatever problems you're likely to see from an anodized aluminum crown tube you'd presumably be likely to see in anodized aluminum bezels/inserts and there doesn't seem to be an epidemic of rust issues there.
That's a good observation about the bezels. Normally steel + aluminum + salt water = rapid corrosion. But decades of subs with aluminum bezels on steel and they're not disintegrating in a weekend.