I do not wish to argue with you, but want to point out that an ISA Arborist Certification is not a license, it is a certification. A license is a legally required credential to permit an individual to perform an action. A certification is a notice that an individual has a certain level of knowledge. Big difference between the two. A certification can be required to obtain a license, but the converse does not work.
I am a licensed commercial pesticide applicator. Also, a licensed commercial driver, and a certified electrician. If I wish to work as a truck driver or a pesticide applicator anywhere in my state, I pull out my license and away I go. If I wish to work as an electrician in any major city, I must first obtain an electrician’s license, as my certification does not give me legal permission to perform a service.
A Certified Arborist who is doing consulting, and doing it properly, likely has spent years in school to obtain the knowledge (and a degree to prove it) necessary to perform his job. A climber likely has not. He may (quite unlikely) have as much knowledge as the consultant, but he does not have the credentials to give him the standing to use that knowledge as a primary means of earning income.
My data for the danger of roofing versus tree care comes from the federal government, by way of the insurance industry. According to the Worker’s Compensation Bureau, roofers have more frequent accidents than do tree trimmers. This is why roofers pay higher worker’s compensation rates than we do in the tree industry.