Is a plus sign necessary in a bounty label?
Or there is a hidden plan to offer a negative bounty?
It's not strictly necessary, no. There is no way to award or earn a "negative" bounty, and I'm not really sure what the application of such a feature would be if it did exist.
But it's not really hurting anything, either. It provides a bit of extra clarity about what the little label means: By answering this question, you stand to gain a possible reputation boost.
I really have no idea what we would stand to gain by removing the
It's what tells the SO/SE newbies that there is extra rep involved. It's a one-symbol communicator.