That right there is the key vulnerability. I'm not familiar with the stations, but I'd guess that the ones with the highest ratio of parked to foot traffic are the ones with the most thefts.

This is definitely the kind of problem that needs to be solved by the BART administrators and/or the local police. If you are parking your bike in a place where thieves know there are a lot of bikes and not enough security. You can try to improve your personal security solution, but there isn't a lock in the world that can't be broken in 8 hours.