Counter-Strike: Global Offensive is currently part of Steam's summer sale; you can purchase a copy of the game for $7.49 instead of the normal price of $14.99. You won't, however, be able to buy a copy for a friend.
So what gives?
"CS:GO will not be giftable during the sale," Valve’s Ido Magal explained via Reddit. "Our goal with sales is to grow the community and historically, during sales, the new users that stick around are mainly the ones that purchase copies for themselves."
Growing the community is certainly one aspect of this decision, but not allowing giftable copies has many other benefits as well. "This is interesting. It will also partially prevent cheaters and trolls from stacking inventories full of gifts to use later," another user wrote in response. "The lazy ones will be put off by the need to create individual accounts to buy the game."
This is a non-harmful way to attempt to help the community grow while closing a few loopholes that could possibly be used for abuse of the system, but it's worth noting that other developers may not have the ability to remove their games from the gifting ecosystem. I checked with another indie developer who could neither find the option to remove the ability to gift their game, nor could find any mention of the option in the Steam documentation.