Why should I go through the purchasing process to install a product priced $0.00?

Usually, free software end up in software-center through Debian repository import but there are special channels to get software directly into Ubuntu repository.

The reason why some products end up with a price tag of $0.00 but have to be installed through a purchasing process is because they were added directly to the Ubuntu repository through the myapps.ubuntu.com portal. Because of the interface of the portal, the only way to release the product for free is if you set a $0.00 price. Hence, you see a buy button for products even when you don't actually have to pay anything for the product.

This issue has been notified and the design spec of software-center has been modified to sort this issue.

A bug report has already been filed to rectify this problem and hopefully the Q-release will have this bug fixed.