Hey Riot Before Making Buffs to Kog'Maw

Think about your past philosophies with other champions first. I noticed while playing Kog'Maw in an A.R.A.M. that his W cost mana.

Let's take a trip back to patch 4.4. http://na.leagueoflegends.com/en/news/game-updates/patch/patch-44-notes#patch-tristana

In patch 4.4, you removed the mana cost on Tristana's Q, because, as a simple auto attack modifier, it is not a tool that you should be worrying if you have the mana to use it, and that you should be able to use it at the right time and not with the right mana.

So can someone explain why Kog'Maw W has a mana cost? I understand that it is better than her Q but it is still just an auto attack modifier.