I think if anything you're stealing from yourself, robbing yourself of the opportunity to take on more responsibility, grow faster, etc.
>You are stealing from your employer.
Lol, no. I get paid $X to do Y work. I'm not about to do free labor for Z when I complete Y. If you want Z done, I need to be paid for it.
Because I'm salaried and paid for my work not my time. No other justification is needed.
If your boss is happy with your output, then it doesn't matter how many hours you work to make that happen.
You are paid to produce a result, not to warm a seat for 40 hours a week. I'm sure you will have all sorts of mental justifications for why that's unreasonable or why others should be working themselves to death for nobody's benefit but their boss's, but that's how it is.