My brother and sister-in-law work for BCG. There are no secrets about what you're signing up for when you take a job there, and they're paid a premium for it. Their clients pay for fast results, and the best way to do that is have employees work long hours. "We have smart people working long hours on your problem" is their entire business model.
Every single person at BCG could easily find another job that turns off at 5:01pm if they wanted to. They choose not to.
From a quick Googling of BCG salaries for Principals, I'm a little shocked at the (relatively) low rates that people are willing to sell all their leisure for, esp assuming that they're generally talented white-collar workers. Though the caveat is that this was a cursory search: the data could be bad or hiding details like TC vs salary.
Every single person at BCG could easily find another job that turns off at 5:01pm if they wanted to. They choose not to.