This is how I've always explained to people who constantly work overtime in salaried positions where they don't actually get paid for that overtime.
You can at look at it one of 2 ways.
1. Your billable self stopped at the 8-hour mark on a given weekday and every hour after that, including weekends, you're just giving away free labor to the company. Just think about that every time you're working overtime, every minute after 8-hours you're freely giving away to the company. This is your personal free time.
2. Or, like patrickdavey said, you dilute your hourly worth. Say you're a dev who makes $90K, that's roughly $43.xx/hr for a 40 hour work week. But if you're actually working 50hr work weeks you're now worth $34.xx, and so on. The company just saved money on you (or they are making money off you).