Reading through the responses to the post you linked to it would seem that a rookie agent wrote a policy that he should never have written since State Farm doesn't cover any companies that manufacture any goods they deem to have a high risk potential. They are mainly a personal liability insurer. Could State Farm have explained it better in their letter, yes they could have but it doesn't change the fact that they do not cover any manufacturing whether it be firearms or bubble gum machines that they consider to be a high risk. They do provide small business coverage for business that provides a professional service and that may be where the problem started since Alex Pro Firearms is a small business that manufactures/customizes firearms.
Does State Farm suck as an insurance company, that's for you to decide but in my dealings with them the answer is YES and there are plenty of reasons to hate State Farm, this just isn't one of them
Because they are not in that business and do not have the information necessary to create accurate actuarial tables which would allow them to properly assess risk and charge an accurate premium for the risk involved.
Commercial lines carriers do this all the time and have the information.
Why doesn't Walmart sell construction equipment? It's not their business.