What we now know is this... Back in the 70's a guy named Ronnie James Dio, formerly of Black Sabbath, formerly of Rainbow, formerly of Elf and now in his own band aptly titled "Dio" was on stage and did it. He continued to do it at successive shows and it became his trademark until it caught on and is now seen on rocker hands held high all across this lovely world of ours.
The only thing that still remains a mystery is the why of it... why did he do it the first time. That we may never know. ;)
I bet these guys seen at a recent Pennywise show just do it they think it's cool... except for the renegade in the bunch who chose to shoot a bird for the camera instead.