Actually to dump is not the purpose of the bounty campaign. It is to spread awareness and if possible hype to the project. It is one of the marketing strategies of any business that opt to fund them through people's money, .a.k.a ICO, IEO, ITO, IPO, etc.
If we follow this comment that means the OP is right then. Bounties cause awareness and that cause more people to know about a product that though listed on exchanges but was never popular. When more people know about it and get interested it will increase demand for it. The price will go up and then the team will sell ahead of bounty distribution and get out, causing price to crash.
The major reason why several projects get dumped the moment they get in exchange is that many of these projects offers discounts on investors. Like their regular price is x USD but they give it at the price of x/3. So who is in the right mind won't sell if they saw 2x 3x profit? Thus, dumps happen.
You are right here and I do not know why projects still offer such crazy discounts to early investors. It is like deliberately decreasing the value of their tokens.