In a 9-0 Supreme Court ruled that Amazon wasn't required to pay their employees for the time spent in a post-shift security line because it wasn't a "principal activity." In other words, if the security checks stopped, their jobs would remain the same.
With a 9-0 decision, it's clear that this is the law. But, just because you can do something doesn't mean you should do something. This wasn't a decision that hinged on the constitution, but rather on statute. Congress can change this if they wish. Regardless, you should not be doing a happy dance and say, "Woo-hoo! Now I can pay my employees less!" Here's why.
It destroys work-life balance. Any time you require an employee to be somewhere or do something, you're taking away time from their personal life. Some employees claim that the security lines were up to 25 minutes long. That's a huge chunk of time your employees are stuck at work when they'd rather be home, and not getting paid for it. Amazon disputes that and says the time is much shorter. Regardless, 25 minutes or 5 minutes, it's time that your employees are required to be at work rather than home.
It attracts the wrong type of employees. Or rather, it discourages the right type of employees. Some security is, of course, necessary. But, when your security procedures not only take up a good amount of unpaid time, but indicate that you don't trust your employees, trustworthy employees will be discouraged and seek to leave. Untrustworthy employees will want to leave as well, of course, but they will be less likely to be able to find a new job. Good employees leaving, bad employees staying is not the type of environment you want to have.
It demonstrates management failure. You can't effectively screen out new hires who don't steal. You can't set up a security system that is quick and effective. You can't develop a workforce that trusts you when you show such blatant distrust of them. The whole system screams, "we don't know what we're doing." And while you're showing you distrust almost all your employees, it puts a tremendous amount of trust in the employees doing the screening. What makes them so different from the others?
Paying is the right thing to do. Even though the entire Supreme Court agreed that this was how Congress had written the law, legal doesn't mean moral. The moral thing to do is to pay people for their time, regardless of whether it's a core function of their job. If your company is requiring employees to do X, you should be paying for that.
This is a narrow ruling. Employment law is crazy and hard to follow. Amazon won this hands down, but other cases will not be so clear. When you try to save money by cutting small amounts out of the employees' pay, you may end up losing in court. Furthermore, the costs of going to court are tremendous. Amazon claims that security procedures only took a few minutes. It undoubtedly cost them millions of dollars to take this case all the way to the Supreme Court. Is that saving money in the long run?