"Willingly" is basically just peer/manager pressure. "Everyone else is doing it, why aren't you?" "Do you want to be the reason we fail to meet our deadline?" My experience in tech outside of video games is that although there's an offer letter with terms and a bunch of stuff like signing invention agreements and non-compete clauses, it's also pretty much always "at-will" on both sides without any sort of contract. The actual job is basically just "do what we ask", with the conditions that each side is wiling to ask for or tolerate being a game of chicken (that tends to favor the employer, since most employees can't survive without a paycheck/health insurance, but an individual employee is likely expendable for the company as a whole).