If they decide they don’t want to pay for the full version, they can cancel their subscription before the free trial ends and avoid any fees.
According to the letter of the law of Google’s policies though, canceling a free trial and uninstalling the app in question are two separate events. Most developers interpret an uninstall as a cancellation of the free trial, but not all. A few unsavory developers have decided that unless the user specifically cancels their free trial prior to uninstalling the app, they’ll charge them anyway.
In many cases, the charges are modest. However, the worst abusers of this policy have hit unsuspecting users with charges amounting to hundreds of dollars for very simple apps like calculators, GIF creators or QR code readers.
Since being alerted to the issue, Google has removed more than half of the offending apps, but a few of them remain. Be sure you look closely at the terms of any app you install. The last thing you want is to be hit with a hefty fee for what amounts to a moderately useful app.
It should be noted that the apps in question can, in no way, be described as malware. They’re perfectly innocent apps that developers have built excessive, even predatory fees into. Kudos to Google for taking a stand against the practice but we wish they had washed them all away. As it stands, there are still a few bad actors on the Play Store, so be mindful of that.