Previously they mentioned about it in the rules, but recently we have not seen such rules because most of the companies are choosing for IEO in lower value exchanges. Most of the companies failing to raise the money and which it is impossible for us to get paid from the companies. So there is no choice for us and we have to look at other potential bounties.
So the reason most wil now prefer to join the campaign that pays ETH and BTC even when the company has a background of scams like the p2pb2b exchange that was about to start a signature campaign. Campaigns paying tokens are just not worth doing with this type of market condition. Hard times to earn BTC, a dim light in the future if we can't collect btc today.