If I'm being compensated straight salary, I'm not invested, and I'm not going to be working heroic hours or making sacrifices, no matter what my employer might wish for.
And here is a truism: Even people who are 'invested' in their jobs are looking for something better, with 'better' being defined as more pay/equity, flexible hours, meaningful work, etc.
If you want to employ someone who is just as invested in the company as you are, then you should compensate them just as well as you are.
What kind of employees do you want, then? Loyal ones? In my experience, the most loyal employees are also the least productive.
Likewise, I tend to think that the ass-in-the-seat 40+ hour workers tend to correlate with the "always looking for a better opportunity" workers.
If we're in a sprint, or shit hits the fan, or revenue is down, or anything else that requires hands on deck, I expect people to complete the task, even if it means more than 40 hours.
If a situation arise because of bad planning an employe are expected to donate their time, but if an employee finish the sprint early the company will find something extra for them to do.