prior to DLC and the Internet, we got the game... that's all, thats it, that was the whole package, you want more, buy our expansion, you want more for free, do it yourself.
With the rise of the Internet, we got nice little patches, they didn't add content, they just fixed bugs and issues, they made some games playable when before they weren't, and yet, people complained that the game should have been perfect out of the box, they complained that they had to download something to fix their games.
Then the modding scene started budding and people got used to the idea of being able to expand their games beyond the experience they got right out of the box, developers began supporting these efforts by releasing SDKs and information pertaining to the game.
Then some Developers decided that they would release their own additions to the game afterwards, developers doing this knew they would take a hit, but in the hopes that it might drive a few more sales they persisted, the modding community for most games was horrified because it's not exactly easy for a few armchair programmers and designers to compete with paid professionals, but eventually it was accepted.
Today, developers know that their customers are willing to spend a few bucks on those additions, this allows them to devote more time to post-production additions to games that would have never seen the light of day in the past due to budget constraints.
It is up to you as a consumer to decide if those additions are worth the price they are asking for then, if it is, then the point is moot, if not, then you vote with your dollar and don't buy it, it really is as simple as that.