That did seem bizzare to me. I suppose it was to stop you from heading back though the tops everytime you wanted to talk to him. It's also possible to get chucked out of the casinos for good (as far as I know, I haven't done any gambling yet) which would mean he would be impossible to access. But yeah, anyone could have talked to him. And why don't the sercuritrons/Mr House get suspicious of him just standing there smiling away? Really there should be a dialogue option that says "Maybe you should find a better place to hide" after you talk to him in the tops if Benny is gone, allowing him to go and find himself a sealed bunker out in the wastes somewhere. Or maybe a high enough science skill would allow you to reprogram him to only help you.