If, in both cases, the rock was the size of the pebble, I would say that the problem is with the design and manufacture of the windshield or louvers. If the rock was the size of a baseball, then nope that is accidental damage. Put it this way: Would you expect your car to break going down an "average" dirt road at 20 MPH? No. Absolutely not. If something hurts your car going down a normal dirt road (pebbles, bumps, normal washboard) then I'd say that what ever broke was not designed properly.
The louvers are in a spot that they should be strong enough to handle "normal road debris". I wouldn't expect them to handle a thunder-bunny like what tangled with my other car, but on the other hand a small rock or something like that: Yeah. They should be made out of heat and UV resistant fiberglass reinforced plastic if they are being subject to normal road conditions. Would you expect your bumper cover to crack because it was hit with a pebble? (sure, if the rock is the size of a baseball all bets are off, but something the size of a marble shouldn't break something on the front of the vehicle).
Here's where I'm coming from: I got this P1D73 code today. I drove ~50 miles from work to home. The last 2.3 miles are 45 MPH or less. My last quarter mile is 10 - 20 MPH.
Seconds before I got to my driveway (we're talking 75 feet from my driveway) my CEL comes on. I cruise into the driveway gently, stop the engine, put the ignition to "On" without starting and read the P1D73 code. I go inside and look it up and get my other car ready to take to work tomorrow. By then it is dark outside and only then did I know what P1D73 means thanks to you! Seriously, Thank You, Fnky!!! I really appreciate you and the information you share!
However, to address your point: In the daylight tomorrow, if it turns out my louvers are damaged in any way, yes I expect them to be replaced under warranty. I absolutely did not hit anything on my way to work or on my way home today. Not so much as a "dink" "tink" sound. I was not behind anyone. Traffic was very light. If something that small can break the louver then it needs to be made stronger. If I hit something large, then I absolutely expect to buy the part myself and install it. It all comes down to what is reasonable.
I've owned 5 Chrysler vehicles. 1 of them had a sun visor that broke in the first few weeks of owning it. It was a plastic formulation problem. By the time I owned that vehicle numerous other people already had problems with it and Chrysler had a newer stronger clip that never broke again. So, yes, there are things that break under normal conditions that should not have broken which should be covered under the warranty. There are also things that break under abnormal conditions and those things should not be warranty issues.
The Crapinental Tires that came with my original Magnum RT died in less than 20k miles. Considering I drove 80 miles a day, highway, to work and back and I kept my tires aired up properly, there is absolutely no way they should have died that soon. So, yep, you can bet that was a warranty issue, too. I wasn't alone in that situation either.
Just in these 5 vehicles I've had (4 of which I still own now) there have been a number of issues that could been looked at either way, but in the end they were all warranty issues because of bad materials to begin with. Could these louvers have the same problem? Not sure, but it's not outside of the realm of possibility.
Now, go right ahead and flog me for replying to you on something 2 years later... I totally deserve it.

But keep in mind: I did do a search to try and find the info I needed before I bugged the awesome and incomparable Mr. P. Have a good one, Mr. P.