A federal judge ruled on Tuesday that Amazon is liable for billing parents for unauthorized in-app purchases made by their children. With the ruling, U.S. District Court Judge John Coughenour sided with the Federal Trade Commission in its lawsuit against Amazon for failing to get consent from parents for in-app purchases made by kids.

"Many of Amazon’s arguments improperly assume a familiarity with in-app purchases on the part of consumers," the judge said in the ruling. "For example, Amazon cites to a case determining that a ‘reasonable Amazon customer is accustomed to online shopping,’ but online shopping and spending real currency while obtaining virtual items in a game are completely different user activities."

The court has not yet ruled on how much Amazon will have to pay out to customers affected by the practice. The FTC previously settled with Apple and Google in similar cases, resulting in more than $50 million being returned to consumers.

"We look forward to making a case for full refunds to consumers as a result of Amazon’s actions," FTC Chairwoman Edith Ramirez said in a statement. An Amazon rep did not immediately reply to a request for comment.

When Amazon started offering games on Kindles, it did not require that a password be entered to authorize an in-app purchase in games otherwise labeled as "free," according to the ruling. Amazon eventually added password protection for purchases of $20 or more, but that still left plenty of room for unauthorized purchases by children. Over time, the company added more parental controls, including a program called Kindle FreeTime.