Unfortunately, that's the way the content owners have decided to do things. It's not just ATV that's affected. You'll find that's true with most streaming media devices. In addition, even if you have a paid subscription to a service like Hulu, there are certain programs you can't watch unless you also subscribe to a cable or satellite provider with that channel in your package (SyFy for example).
For what it's worth, I jumped ship from U-Verse for TV to DirecTV. AT&T bought DirecTV specifically so that they could kill off their U-Verse TV service. It was never profitable and it was affecting their ability to deliver higher speed Internet services in the markets where it was available. That's why they stopped offering the promotional deals they used to have. I had dropped down to broadcast only service, like you, and had kept the DVR. I found that if I kept the AT&T Internet service (which I did), I could get DirecTV for a fixed price of $50 per month including all the equipment and DVR for 4 TVs for 2 years. That's only about $3 more than I was paying for broadcast only U-Verse.
The wife complains when the signal drops occasionally during really bad storms, but I just remind her we could just go back to broadcast only service if the likes... that usually ends the discussion pretty quickly.
Other than that, it's been great. The cable companies in my area couldn't come close to matching the price for the package.