Because it's the kind of thing high-salary workers do often without even considering it stealing. The headphones are used. If someone has a bougie enough job, they probably expect not to be assigned used headphones, and they don't think through whether the company would be selling them on because they certainly wouldn't bother in their own life. Alternately, there is a mindset of "oh, well, it'd be more money to clean them than they're worth at this point, they're just going to throw them out" (mentally estimating from the effort at the rate that that person is paid, not at the rate earned by the person who'd get stuck with the cleaning). Personally, I've seen people do this in ways I find shocking. The author is pointing out that she has also been affected by her environment, that she also has changed to act in ways she would have found unbelievable prior. I don't think it's trying to say "look I've advanced to act like these people".