Because they use the term blockchain with it, providing anonymity and because they can easily get away with it. There are no means to protect oneself aside from reading the whitepaper, checking their website, Telegram, Bitcointalk announcement, and other details. When others invest in ICO, those are the things to check if you're going to invest your time in bounty programs. So for me, I don't just do a single bounty to even out the playing field if in case I stumble upon a fraud bounty.