Firstly IEOs on centralized exchanges doesn't do bought campaign because the exchange like binance or huobi handle the tokensale by supervising the sales on their launchpad for the project owners so there isn't need for bounty. IEOs on Dex do bounties but are not guaranteed of success like airwallet having problem with Trebit
If you are looking more closely, you will find IEO from big exchangers do bounty campaign. I dont think doing bounty is a mistake for project because its used for marketing and i am believe doing bounty for project campaign is good to spread the word about project