I think the original intent of a locking jack was more about the musician accidentally stepping on their own cable and then pull it out of the jack. It certainly isn't to pull down a stack of amps, trip other musicians, or dangle off of a mountain. How many people here had a locking jack save their asses during a performance or in other situations? I'm curious whether it's as useful as the person who designed it think it might be.
I'm sure that was the original intent and even if that is the case, It typically is only going to address that case when the jack is normally down-facing. If I'm not mistaken, on the EHBs it is more rear facing so unless you are holding it up vertically, personally stepping on the cable would not pull at an angle that pops it out.
My guess is that some factory parts manager was looking to unloads these and Ibby figured they could justify the price better with "added value"
USPS tracking is always a little vague