Here's what it most likely is:
On Friday Blizzard decided to crack down on people AFK botting. It's gotten really bad over the last month or so and it's one of the main focuses of rants in the general and pvp forums.

Before this, Blizzard basically did nothing. Players could report people AFK or boting, but nothing happened other than the AFK debuff. A friend of mine had been using a bot for the last few weeks almost on a daily basis. I knew he was getting reported constantly because he would come on after work and be like "aww, i only got 1500 honor in 10 hours???". Other than that, absolutely nothing happened to him, although I'm sure blizzard was building a complaint file.

As of Friday he got the exact same email, but his was legitimate since he was really botting. I'm pretty sure all the emails Blizzard sent out were based on large amounts of previous complaints. And since Blizzard never did anything about it, there was no way to know that what you were getting reported for.

Now that they decided to take action, they probably looked back on your file and said "oh, he has XXX number of complaints that said he's botting. Since they were reported long ago, there's no real way to check if they were valid... but since so many people complained there's a good chance that they are."

So, they decide to issue a warning. I think now that since Blizzard has set a policy to take action, they will investigate more thoroughly before taking action, and you'll be able to make your case. You probably just got targeted as part of their giant "okay, we are going to do something now" campaign.