I think part of the issue is the way FB allows you to use 'dark' posts that are hidden from public view, and also to set up multiple anonymous pages for your ad campaigns.

Together this means you can not only use laser targeted advertising, but show ads to one group you know will appeal, without a second group who would be put off by the message ever being aware of it.

There is also the potential to break rules on election advertising and false claims as the only people to see your ad are the small, well targeted group and an FB reviewer who is checking the ad against their own T&Cs, not the electoral rules of the country the campaign is in.