Zhukov said:
Umm... pretty sure the Battle Bunny skin is just a joke.
Asking why Riven would fight dressed like a playboy bunny is like asking why Poppy would bludgeon people with a giant lollypop. Answer: She wouldn't, it's a joke.
I feel like I should answer your question. So I will.
Battle Bunny skin was made mainly because of the LOL community suggestion.
It was first suggested by an infamous forum troll called Shaella under the name of "bunny girl Riven".(god bliss him, he made LOL general discussion so much better while he was active)
Bunny girl riven post received one of the highest upvotes(equivalent of "like" in Youtube or Facebook.) in the history up to this date, and later got removed by Riot.(understandably so.)
This is the original content of the Original post according to my memory.
Riot should make Bunnygirl Riven skin because:
1. Riven jumps around like a bunny.
2. Because it will sell well.
Make it happen Riot.
In short:
Yes. It was a joke.
While I was trying to search on google to find how to spell "Shaella", I stumbled across on this post posted by Shaella himself.
Read it if you want to, or whatever.
http://www.teamliquid.net/blogs/466620-why-its-so-easy-to-fall-into-the-sjw-trap
So yes, Let's all blame Riot for doing what community wanted. Because reasons.
p.s: Oh, but you can still go for french maid nidalee or police caitlyn skin. Those are probably from Riot.
Edit: Grammars. Why are they so difficult?
Edit 2: Ah, It was all fun and games back then. Now it is all about.... THIS. This is making me depressed.