This is just plain not true.
There is no special line on the 1040 form for bonuses. They are taxed as ordinary income.
Now, because they are in addition to regular income, they are taxed at your marginal rate, which is higher than you average rate, but there is no special treatment of bonuses.
Either way, if a high-wage earner earns a bonus, he will donate 40% to the government. The same for restricted stock that vests. If it quacks like a duck, it is still a duck.