The hanging around garbage dump advice cracked me up... although it's very true! I still don't buy the other story (bear attacks instigated by periods) 'til I see proof - although while we are on the subject I have never hiked in grizzly territory and they seriously unnerve me!
And you are right: theoretically a portion of women would be menstruating at the time of an attack regardless of cause.
Also... so theoretically we are worrying about the cup... and bears have a mind-blowing sense of smell... if you're going to opt to be concerned about the clean, bagged cup... you're going to have to be even more concerned when you are actually menstruating!