It's something hard to pull off and not really effective because the address is brand new (no transactions to trace if it was marked as a scammer's address), and the scammer can also easily generate another address using HD wallets like Metamask or Myetherwallet when applying for another campaign using the same usernames
Actually you are right but reporting it would let anyone know that their address has been used for scamming. Its gonna be hard to catch these guys because they are just depending on the assumption that the account they used are also joining the campaign they trying to get some tokens too. Maybe some manager missed it if they were careless and too bad the one work hard for pormotions only scammer will received their rewards.
What I can think the reason behind it only the campaign manager didn't work properly or handled by an unprofessional manager.
Correct. If those works have been credited then only scammer will gain the rewarda, but since they are randomly doing that they are also not sure but there are cases probably that tokens went to scammer address instead of the one work on it.