How can this be allowed by their merchant agreements with the card companies?

Whether it is or isn't, threats go a long way.

Like the signs you see in parking lots that say "not responsible for lost or stolen items" or "not responsible for damage to your car".

They often are, it just says that to cut down on lawsuits.

(This used to be universally true, though it's become less true over time that they are responsible)

"Keep back 500 feet, not responsible for damage" on trucks carrying gravel or other debris / detritus.

Absolutely not the case, and non-enforceable. You just have to be ready to show that the civil balance of probabilities is that the truck ahead of you on the road did damage to your vehicle by way of an unsecured load.

Credit card companies aren't really interested - various businesses will blacklist you if you chargeback against them, more of the same really.

I forget what it was, but a fairly large business was noted for this - I want to say a motel chain or something.

Steam does it too. Status quo.

I'm aware. Question still stands.

Well what exactly does the question mean? I can't think of an answer to how that would satisfy you. They simply agree on the terms.

And ensures that they'll immediately get more chargebacks. Good luck with that.