The actual origins of the term are unclear. What PETA gives is a possible source, but there are several others. It could just as well be one of them. Anyway, it must be a pretty miserable existence to have to dig around finding stuff to be offended about.
In the USAF, 1966-1969, we had a barricade in front of the B-52 and KC-135 engines when running them for maintenance.
It was also called the “Bullpen”.