BBs hold up better than that, so you have to look for causation. It could be weather, but you say you're a fair weather rider, so unless you're always washing the bike rust shouldn't be a factor. If you are washing the bike, and the BB is rusting, then the first thing you should do is stop washing and see if your luck changes.

I understand the frustration, but new bike warranties are usually not portable. It's assumed that buy your bike locally, and therefore returning to the same shop isn't an issue. That doesn't work if you move or travel a lot, and sometimes companies will accommodate that if you can find a dealer in the new location who'll go to bat for you.

Changing mechanics each time doesn't help, though I understand why you must, because each one sees only what he sees, and has no sense of the history. Upgrading to a better BB might help, or it might only mean higher yet replacement cost, so I can't advise there either way.

BTW- being at the wholesale end of the industry, I know many of the shops throughout the country, and if you post the city you're in, I or someone else might recommend a decent mechanic local to you.