The fact that some people play the game for free has absolutely no bearing on this discussion: This is an event that affects people who pay money, and no one else; the only people complaining are paying customers. Bringing up that other people might play the game for free is completely irrelevant, so I don't see why it would be brought up at all unless it was being used as some kind of excuse.
It's NOT a free service. This is a business which is funded by transactions. The fact that the payment model allows lower-level free service (even if that low level happens to be rather high in this case) doesn't change that. A paying customer has every right to complain about services involving payment. The free content has no bearing on that.
Of course if people don't want to spend the money, they shouldn't complain, but that's exactly the problem: EVERYONE HERE WANTS TO SPEND MONEY. We all love the game, we all love the free and the paid content, and we want to play it more and buy more content. Customer service complaints are never matters of people who don't want to be customers; they're matters of people who DO want to be customers, but don't feel they're being treated well.
And I wouldn't even care if it WAS pay to play. I bought Awesomenauts -- another MOBA -- which requires money to play, and I bought the expansion, AND I bought tons of skins that weren't even free with a paid copy. And had I known about the kickstarter for the expansion when it was up, I probably would have helped fund it too, because I wanted to play the game.
Difference between Awesomenauts (via Steam) and what's going on here with League, is that Steam does its savings events DURING Christmas, not after. I bought tons of Awesomenauts skin for lower prices during that event (since Steam doesn't use a points system, just direct money transactions, the pricing is the best parallel) with no issue and no complaint, and I know DotA 2, one of League's biggest competitors, did the same thing though I didn't buy anything from that game.
That's an example of two of League's competitors doing better customer service, so Riot should take notice.