Technically it is unsportsmanlike conduct. There is no explicit foul associated with shooting the break ball early.
Common sense might dictate, if you allow your opponent to shoot before the balls are racked, you have essentially conceded the match. Same with ‘conceding’ the last shot.
Question now: Is a concession ever considered ‘unsportsmanlike conduct’? Though I feel any player obviously has the right to quit anytime, with a televised or paid-attendance ‘championship’ event (excepting illness or injury), I could see where that designation might well apply, though what kind of ‘foul’ penalty would really be relevant if a competitor disappointed the spectators by conceding and walking out on the final?