So when a company spends it's time and resources creating additional content for a game it should not charge the customer?
Honestly, it varies from game to game. It was the Xbox (just last console generation) that made popular the idea of DLC, and what many people either don't know or remember anymore is that most all of it was free.
It used to be that game makers produced DLC primarily as a way to extend the game for the fans, as a gesture of good will of sorts. This was wonderful, in my opinion. Sure, there was some DLC that cost money, but it was much more rare and usually only justified because it added so much to a game, such as doubling the total number of online maps in an FPS game, something really substantial. It didn't cost $15 either (*cough* Activision).
Things have changed in just a few years though as DLC is now seen as a major source of revenue for game companies. The era of the 'freebies for the fans' has pretty much been totally forgotten, to the point that people such as yourself actually seem to approve of being nickel and dimed to such an obscene extent. This is not an attack on you though, as you do raise an interesting and debatable question.
My answer is just as I stated above, it varies from game to game. Personally, I don't mind paying for DLC occasionally, so long as the company is being reasonable in their approach. In other words, asking money for horse armor (Oblivion), $15 for pack of old maps (MW2), or (worst of all) characters in a fighting game that have purposely been taken out of the series roster and have already been announced months before the game is released that they'll be sold later as DLC for increased profits (Marvel vs. Capcom 3), is a lousy way to treat your customers. On the other hand, if a company like PD (Sony) wants to charge a reasonable amount of money for new premium cars in GT5, I'd honestly have no problem with that, simply due to the fact that they have been so great (my opinion) in responding to problems and providing more content (such as online events) for their fans already.