Scammers are just targeting random users of this forum who seem to have a large amount or even just a little BTC. They would get negative trust if they post it in public(and they know that), so they go through the method of PM'ing.

Well we're already trying our best to prevent this from happening by implementing the 360 second rule for newbies... So they have to wait 6 minutes before sending another hoax to another user. That and points of evil.