It's because a lot of them are out of stock. That $15 price is from third-party sellers.

Prime members are still able to get them 50% off, despite them being out of stock though, it seems. For example, look at Emmet's page. It says $15, but if you click "other sellers on Amazon" or the "24 new for $7.49," you can see the Amazon price for $7.49, but it's tagged with "Exclusively for Prime Members."