Alright,

Since this comes up reliably every few months, imma make comment. THIS IS NOT A DEFENSE OF THE NTTA OR TXDOT, just an explanation to what has actually happened and why its still tolled.

Initially, when the DNT was built, the NTTA (then Texas turnpike Authority) was to use the collected tolls to maintain the road as well as pay off the bonds used to fund the construction of the DNT. This is, in fact, what happened to I-30. It was initially it was a toll road, then handed over to the DOT after the last bond payment. Since then, the budget for TxDOT has only gotten worse. Despite budgeting more money annually that any other state (except for Caltrans), we are in a severe budget shortfall. We have more infrastructure than pretty much every other state, and its not close. In fact, Texas tops the list as the state with the most bridge structures by more than double.

This shortfall is largely due to how road construction is funded. Revenue for TxDOT to build and Maintain our roads comes for a few sources. These are Bonds, registration fess, and gas tax (both federal and local). The gas tax does a lot of the heavy lifting on that funding. Both Federally and Locally, the gas tax has not been increased since the early 90's, hardly keeping up with the raising cost of labor and materials needed to build things. Couple this with the amount of people moving to Texas, and you create a system that is not only struggling to keep up, but actively falling behind. Not only can we not build sufficient infrastructure for the populace, maintaining the current shit is getting more and more expensive.

Because of this, it was decided that the NTTA needed continue to operate in its current state maintaining and funding new expansions, using tolls to do so.

Basically, that was the intent in the beginning, but life got really unexpectedly complicated in 30 years.