I honestly see that they have every right to be frustrated, but IMHO spending time hunting vs. focusing on your content is kinda silly. Also the empty threat is kinda.. cringe.
It's the internet where digital good are sold -- there's bound to be theft... ALWAYS. I've even made digital content in the past only to have it pirated, but it sold plenty copies still. The perspective should be that those who were gonna buy anyways were gonna buy it and I still personally made a pretty penny but in this case they're upset at the loss of potential income. If anything this shows that people like your stuff weather or not you profit and if anything it helps promote your popularity. Free advertising too!
Again, they have every right to but all they're doing by putting that message in their avi's means you're poking a bear and just making people steal your ish, essentially egging them on.
(to the creator) Bro if you're reading this I feel your pain but all you did was ignite a flame dude.