​Why sharing Amazon Prime just got tougher

While Amazon (AMZN) likes to make big gestures (remember Prime Day?), it's just made one major change in a very quiet way.

The biggest online retailer has changed its Amazon Prime policies to restrict the number of adults who can share the service, which includes free two-day shipping and online video streaming. The service costs $99 per year, but some intrepid adults -- roommates, friends and family members -- had been joining onto one account as a way to lower their costs.

The service had previously allowed four adults to join one Prime account, but with Amazon's latest tweak on Aug. 1, only two adults can join one account. There's another critical change that might make some roommates think twice before jumping into a Prime-sharing situation: To get the benefits, all account holders need to authorize each other to use credit and debit cards associated with their Amazon accounts.

That may be perfect for spouses, but it's likely less than ideal for roommates and friends. Amazon, which now calls the sharing service "Amazon households," also allows up to four children to join in the program, although the kids don't need their own Amazon accounts.

Amazon didn't immediately respond to a request for comment.

While paying up for a Prime account may still make sense for many families, the change appears to be geared toward bumping off some adults from a shared account. There's plenty of financial incentive for Amazon to tighten its policies: convincing (or as some might see it, forcing) adults to pay up for their own accounts could pay off with more revenue for the company.

The $99 annual fee may be only a small part of it. Amazon Prime members spend about $1,500 per year, or more than double the $625 that non-members spend on the online site each year, according to a January report from research firm Consumer Intelligence Research Partners. About 45 percent of Amazon customers are Prime members, or more than 40 million people, the study found.

There is some good news for people who have been sharing Prime benefits. If you had been sharing accounts before Amazon made the tweak on Aug. 1, you won't be impacted by the change unless you opt-in, according to LifeHacker. That means that roomies that were sharing free shipping through one Prime account will still be able to do so, and won't have access to the other's credit cards.