Yes it is, for plant workers it is a firing offense in many places.
But your typical maintenance tech that is supposed to work on the machinery itself rather than just to be using it may well use lockout/tagout to ensure that the machine is powered off when they are working on it and don't need to be able to do any kind of diagnostics. But short of a complete disassembly and testing each component in isolation - for which there often isn't really time and which given the pressure on maintenance technicians to get a piece of gear working again - people tend to take shortcuts such as to hook up analytics gear to a machine that is live. Stupid? Yes, absolutely. But this kind of stupidity is the result of usually many years of things working just fine and bad practices creep in. The guy that manages to get stuff done rapidly is the one that gets called out. And eventually an accident will happen. Weirdly enough even near accidents tend to reinforce the belief that it worked. When actually the lesson should be that it didn't but the person just got lucky.
One very memorable occasion at an industrial plant I visited is that someone got beaten up for removing a lock and I felt absolutely no sympathy for that guy. He could have well gotten someone killed (large 5 axis mill).
Roughly halfway into your sentence about the concrete slab my mental kinetics prediction model already declared a zone described by the pendulum at maximum extension (hanging from one remaining strap) as no-go area. And depending on the state of the crane and how far debris could have been shot out from a falling slab that area may well have had to be much larger still. People that don't understand such dangers should not be in management positions, which is one of the reason why I'm always happy to see industrial companies that promote people from the ranks to management rather than to bring in outsiders with only theoretical knowledge.