I did a lot of catwalk work on cranes at Boeing, we were not required to tether up as long as we were inside of the rails, but on or above the rails, yes. Scissors lifts did not require being tethered, but were limited to 10 or 12ft extension (IIRC). JLGs though were 100% tethered. I was test driving a bridge crane once and hit a guy in a JLG when he was 120ft up in the air (I couldn't see him from my carrier carriage, he was not supposed to be there during testing). The JLG teetered and wobbled but didn't fall over (lucky for him) but it did toss him out (unlucky). His harness saved him but he hung there for an hour or so until the Fire Dept. ladder truck with a long enough ladder got to him. I was right there with him the whole time, but they would not allow me to attempt to pull him into the crane carrier with me.
Paperwork on that cost me two days of work, although, I was billing them by the hour...