seems like a very odd requirement and unreasonable. This is America. You should be able to get your insurance any where you want.
The classics and the dailies are unrelated to each other in any way
Actually it is a fairly common practice. Same thing happens at USAA when you try to insure a motorcycle and are directed to Progressive. It has nothing to do with what vehicle is related as you mentioned, but is how the individual company decides to handle the insurance policies as they choose. You don't just get health insurance where you wish either as the same thing applies in that industry too, so being America or not, this is the business practice of the companies and not dictated by the wishes and desires of the people when it comes to how they conduct business. It is not really an odd requirement at all as the companies are in business to "succeed" as they see fit. You may not like it personally and find it unfair, but if so, the other options are to find an insurance company that suits you or start up your own.
Me, I was also directed to use American Collectors from my USAA carrier. Been doing that for this current Model A pickup I own and for my old tudor I had 10 years back. Same company for insurance on both. No claims ever filed and no problems from them in the least, otherwise. Both old Fords were daily drivers and the rules for those are somewhat varied from insurance company to insurance company. I think we are pretty blessed that the states are not so rigid in allowing a vehicle of classic age to even be on the road as a daily driver, myself.
I am not purposely trying to start up an argument in this thread which can turn it political, but stating what you indicated is not correct as to how the auto insurance industry works and how it is applied. Being "fair" or "getting what you want" as an American is not anymore a right in the auto insurance business as any other. You can choose to pick your company for yourself if you don't like it or go elsewhere, but that is about the limit of your choices.
As to how good or how bad this insurance carrier actually is with processed claims is not something I can say as I never had one to file. Same was true when i had Progressive over the years for my sidecar rig. There are differences between companies as to how you insure the worth or value of replacement for your vehicle so I suppose that could be a factor as to which one you ultimately choose. USAA does not force you to choose only Progressive or American Collectors, but they do guide you to them when you contact them to insure your vintage auto or motorcycle. As with anything, research and make your choice wisely for the best deal you can find.