I've always wondered why this is so in competitive powerlifting...
Forgive me if I'm wrong in interpreting how an actual sanctioned meet runs, but all that follows is based on my impressions of having attended a handful and occassionally watching them on the deuce.
If I understand it correctly, when a lifter performs the squat, he unracks the bar, assumes the position, and when all is 'kosher' with the judge, he is given the 'go-ahead' to begin the lift. He (or she) then performs the lift with no further instruction or commands, only scrutiny. After a hopefully successful lift, it is then up to the judges (one in the small meets I've attended...three, i think, in the ESPN2 televised meets) to determine whether the lift was completed satisfactorily.
If that is the case, why is it (apparently) different for the bench press? From what I know of a competitive lift, the lifter unracks, descends, and then has to wait for a signal indicating that there has been sufficient pause. I'm curious as to why that is the case. Given the way that the squat is judged/performed, why isn't the bench press done in a similar fashion with the lifter performing the lift at his own pace and pause with the judges then determining whether or not there was sufficient pause in much the same way that they determine whether a squatter successfully broke parallel?