Why doesn't Riot honor older skin codes?

You can thank hackers and online skin shops.

People were selling them for A LOT of money on le ebays. They were intended to be promotional type gifts for being a cool person and coming to Riot events. I'm not actually sure this is legal of them due to all those gift code/card rules that exists in the USA (California especially), but who knows.

That must be the case, I'm just remembering back to all those huge court cases a few years back where it was basically decided any currency, whether physical, monetary, or otherwise, that didn't come with an indication of expiration date technically couldn't expire, even if it were given for free, so long as it had a monetary value of some sort. It's why gift cards can't expire anymore and must always be honored, unlike they used to be pre-2000ish. It could be argued these free skin codes take on a particular value and, by Riot not honoring them for whatever reason, falls under these cases. I seriously have no clue though, it had always just been a thought in the back of my mind about it all.