It's true that today, few companies give free DLC. This wasn't always the case.
And even today, companies that don't have anywhere near the sales that ND's has had with TLoU, offer free DLC. A new difficulty mode is something you add via a free update. Charging for it, is greedy, as it's a small amout of work on something that isn't actual new content. The fact that they are charging absurd prices for all their DLC, only makes it worse.
As more people buy stuff like this, developers and publishers get bolder. Leaving out content to release it later as paid DLC. Demanding absurd prices, and charging money for simple updates. ND has done two of these things now. How long 'til they withhold content on us? I suppose you can't blame them when people are stupid enough to pay for anything these days, as evident by games like Battlefield and Call of Duty. Perhaps it's a result of a large segment of gamers not having any finiancial responsibilities, and as such, lack knowledge on the actual value of money.
In any case, this effects everyone. Including those who don't rush out to buy stuff without thinking twice about what they're getting for their money. Voting with your wallet means little, when all too many kids get their parents to throw money at whatever they want (I'm not accusing anyone in here of doing that, simply giving an example).
Also worth noting is that the only thing in the 3rd DLC pack that takes any real time, is creating maps. Skills are just a number of values that will of course need some play testing. And guns hardly take more than a day to make.