If it's gravel they are transporting it's obviously their fault, it's the responsibility of the driver to secure the load (with some blame falling on truck companies for providing insufficient equipment).

If it's random gravel from the road it's more understandable. But even then the driver is very much responsible for the mud guards on the truck they are operating, just as the police would write a ticket to the driver for worn down tires or broken lights.

But are they “a punch in the face”-responsible?

I lived in Boston for a while. Cracked windshields were extremely common. No one was ever upset at another person.

I think you're missing the joke. If truck drivers could actually put up a sign saying they are not liable for any debris falling from their vehicle, and have it be a valid defense in court. Then they would just put up a sign saying they are not liable for any black eyes given when they see debris falling off a truck.

They're taking the unrealistic expectation of the truck driver's sign protecting them from doing something illegal and flipping it. In other words "If you coul just put up your own sign and get legal protection to break my windshield, then I could just as easily put up a sign giving me legal protection to break your nose."

This is usually related to drivers who do not use the cover of their truck they are legally supposed to. So rocks fly out the top.

And usually because the truck is over full too. For almost any load, if you fill the truck to the brim you have overloaded it. (Unless you're moving styrofoam)

Or dump trucks, which leak out the seams as they go over bumps

Also mud flaps

Or smaller contractor type trucks with tools in them.

Ever have to dodge an axe at 35MPH? Not fun.

A driver has a legal obligation to not drive a vehicle that is spreading debris on the road, which they are often doing and that debris often comes from their construction sites. There are places that use track washing stations at entrances and exits to prevent this.