cognitively it probably makes more sense. ie Im on the same wage but i can brag Im getting $300k extra. If they offered them market rate and an extra $100k it wouldnt sound so great and wouldnt make as much blog traffic ;)
Think about it this way: on your W-2, (assuming the US here) gross bonus and salary are not separated into two different categories. Thus, it can only be taxed at the same rate.
That being said, I'm surprised you'd say that salary is taxed higher… the typical misconception is to think that bonuses are taxed higher for the aforementioned reason.