I find it interesting where these assumptions come from. They seem to filter into sub cultural millue (and even into wider culture) until they become accepted fact. Viz: ghosts are spirits, orbs are ghosts, ghosts drain energy etc. (Hey it happens in areas other than ghost stuff, they're just examples)
I guess it is some critical mass thing - eg. if enough people in a group think ghosts drain energy it becomes a given. You can then build your castle of why ghosts drain energy on the sand that they do.