Happy Anniversary Riot! I'll be honest and say that I've been a big fan of all the folks at Riot since closed beta. (and the ones who've come along since then are great too)
I will say though, I too am disappointed that the skin is either not on sale, or free all together. (or an alternate skin) While you're under no obligation to do this, it would have been super nice and in my mind, the way to go.
Instead, the perception here is that you thought about it, decided it would sell gangbusters, and offered an audio track instead. While a nice track, I'm quite sure most people would prefer in game content.
The funny thing is, while this skin IS awesome, and will undoubtedly sell well, if you had offered it for 50RP, you would have gotten immense good will and indoctrinated a whole HOST of new people buying RP.