My employer gave out discretionary bonuses, my bonus was half of that of a co worker with half the amount of work, and same level performance, and they have been there less years than I have. Is it legal to give discretionary bonuses with no actual bases,

My employer gave out discretionary bonuses, my bonus was half of that of a co worker with half the amount of work, and same level performance, and they have been there less years than I have. Is it legal to give discretionary bonuses with no actual bases, but just because they may like someone better? When asked about the parameters the bonus so that I may strive to meet this goal to receive a bonus in future it took several weeks for HR to get back to me and state it was discretionary and they do not know why they chose the people and amounts of the bonuses. Company handbook does not state any information regarding bonuses whether discretionary or non-discretionary, is this legal? The person who is deciding this amount of the bonus is someone that I have never spoken to, no have I seen them on our organizational chart.
I raised this question after finding out a co-worker received more than double my bonus when their annual review was 1/10 of a point higher than mine, and all of this after filing a compliant with our independent ethics board.