I'm fortunate enough to receive a (nearly) regular annual bonus at my company. It's based on a formula of financial incentives and level within the company. Once the full bonus is calculated, I have to multiple the result by 0.6, to arrive at the net payout.
You're absolutely correct. Why is such a bonus considered special enough to take 40% and give it to the government?
It is not taxed at a special rate. It just looks that way because it is incremental income and is taxed at your marginal rate. Your normal paycheck appears to be taxed at your average rate which is always lower because we have a progressive tax system.