What's the point of a paywall?

So, this may not be true for all online newspapers but what's the point of forcing people to pay for an online subscription when there is always some "free" outlet to get the story?

Case in point. I'm on the RSS feed for the Dallas News. I added it when the online paper was "free." Now, I mainly see the headlines and a few lines of the story before it expects me to "get Subscriber content" upon opening/paying for an account. Anyway, this story was was of the headlines in my Dallas News RSS:

Wylie man's invention turns trash into building blocks for Third World homes

I was more curious about it and I'll be damned if I'm paying for "news." So I Googled it. Turn out NBCDFW.com had it for free (here).

So again, why charge people for what is essentially able to be found for "free" somewhere else? It's not a very good or smart business model...unless, you bet your money on those idiots that will pay for it. Which I'm sure they did, there are, and that's why they do.

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That's all I got to say about that!