Apple's service allows users to pay for subscriptions to digital magazines and newspapers via its iTunes app store, with Apple taking a 30% cut.
According to the Wall Street Journal, the US Justice Department and Federal Trade Commission (FTC) are at the preliminary stages of examining the terms set for media companies as some legal experts believe these rules pose anti-trust problems.
The terms prevent media companies from linking to external sites through apps, thereby preventing publishers from advertising alternative subscription deals.
"Apple's rules don't stop media companies from selling digital subscriptions on their own. But the company imposed restrictions that could make that option less attractive to customers and steer more sales through its own system," said the WSJ report.
The European Commission is also believed to be monitoring the situation.
Google also launched its One Pass digital subscription service this week. Google said it would take a 10% cut of payments.