subpoena for court

Do you have a written employment contract the specifies the terms of your bonus payment? It is guaranteed under the terms of a union/collective bargaining agreement? If so, they you can file suit; if not then you have no recourse. This is true even though your absence is due to your being subpenoaed to appear in court. While the subpeona obligates you, is it not binding on your employer. Bottom line, a company can set the conditions of employment much as it sees fit (absent some form of legally actionable discrimination).

If you have a written bonus contract or agreement, they must pay it if and when, and in the amount, provided for in that agreement. If they don't, they would be in breach of contract and you could sue them. However, if you don't have a written bonus agreement--or if you do, but it by its plain terms gives them discretion (flexibility; makes the bonus subject to their judgment; etc.) over when and how much to pay--then they can do this. Except as guaranteed by the plain terms of a written contract, a bonus is at the employer's discretion, and they could choose to not bonsu you if you miss work for a court case; the subpoena does not obligate them to pay you any discretionary amounts.