The reality is that you and your dealer have no say in who is held responsible in these cases, it's the insurance companies who make that determination based upon the laws that are applicable in your state. Most dealers (including us) have a waiver that you sign when you take a demo ride, which states that you are assuming responsibility for your actions and most importantly that you understand that the dealer is not liable for damage to YOU if you crash (medical liability, which is far more expensive than the cost of a bike). Most of those waivers will say that you are liable for any damage you cause to the bike as well (you break it, you buy it, etc.) but in reality that's more just to make sure you understand it, the actual legal liability is based upon state law and not the waiver. Even if you didn't sign a waiver, you're still responsible if state law says you are.
So let's say you either did or did not sign a waiver, you go out on a demo ride, and you crash the bike through your own fault. The dealer will turn that crash in to their insurance, they will cover the claim, and then they will contact the rider's insurance company and the legal department of each company will pour over the policy language and state laws and determine which company is legally the primary coverage in this case. If the rider's insurance company covers it, then they reimburse the dealer's insurance company for it. If the rider's insurance company does NOT cover it, then the dealer's insurance company will go after the rider to be reimbursed based upon state law assumption of liability if that applies in your state. I can't speak for anything other than Nebraska, but here in the land of corn the rider is going to pay for this, whether they or the dealer really like that or not, the insurance company will sue to get their money back if they have to, and you will pay. Now if the crash was caused by another driver, then there is a 3rd insurance company involved who may be paying for it. The bottom line is that the dealer's insurance is not some kind of free ride that lets you crash a demo bike and not have to pay for it.