Why don't some games have Add To Wishlist button?

Why is there no Add To Wishlist button for some games, such as Borderlands 2 (as of 2013-12-28)?

Steam will always show the Add To Wishlist button on the Store page, quite high on the right sidebar, unless

• you are logged in with an account that owns the game or
• you are looking at a package or free-to-play title.

In the case of Borderlands 2, you either own the game with your current account, you simply missed the button or it's a rare bug.

As with Borderlands 2, this seems to be a bug since the "add to wishlist" button is normally there. However, on Steam bundles there is no such button because they change over time and can get removed in favor of new ones.