"The contractor takes on a project for a fixed price to install a
tub/toilet/sink, etc. that are supplied by the buyer. By doing so,
the contractor assumes risk of damaging material. Any contractor that
does not factor in a contingency for possibly causing material damage
is making a mistake. "

You think it's a simple open and shut case that the contractor "caused"
the damage. How does anyone know this? To start with, you've only
heard one side of this story. Second, when working with natural
materials, I've seen stuff that looked fine to start with, but turned
out to be less than satisfactory. Haven't you seen a piece of lumber
unexpectedly split on you? Would that be a carpenter's fault for
causing damage and should he pay for it, even though he didn't provide
the wood? I would say it is something that can be reasonably
expected to happen on occasion and it's not the carpenter's
responsibility to pay for. To lay the blame on the carpenterr, you
would have to prove that he was in some way negligent.

In the case of the granite, you are jumping to conclusions. No one
knows what kind of granite this homeowner bought, where they got it,
etc. The vast majority of people buying material somewhere and then
having someone else install it are doing it because they want to save
money. So, who knows what kind of crap they bought or where they got
it. Granite is common enough and there are enough local vendors that
would do the whole job, that it's very likely saving bucks was the
motive here too.

Here's a story of a contractor allegedly causing damage. My father was
a welding contractor. On large diameter water wells, like for
municipalities, instead of screwing lengths together, they butt them
together and weld them as they go in the ground. On a job many years
ago, after they had gone down several hundred feet, the pipe already in
the ground broke. The well driller blamed my father because he welded
the pipe together and was going to sue.

My father contacted Marquette, who made the welding rods he used. They
flew in two engineers to take samples of the pipe which were sent to
their lab for analysis. Their metalurgist's conclusion left no doubt.
The welds were perfect. The pipe on the other hand was made of cheap
crap import steel that was brittle and unfit for the purpose.
Confronted with that, the well driller folded up his tent. Following
your logic, my father would be the one clearly to blame though, because
he "damaged" it.