Makes no sense that sending an appraiser requires you to use an approved shop.
Approved shops - which basically means a shop on their list who does repairs for that insurer at a discounted rate, cheaper parts, and possibly less-thorough repairs - typically write estimates for the insurer according to the insurer's guidelines.
That includes photos, sourcing the cheapest parts, and doing much of the paperwork; the original intent was to eliminate the expense of an appraiser.
It has largely morphed into something less than consumer-focused, and in some cases, sub-standard repairs.
It's called DRP - Direct Repair Program - and puts the shop's focus on appeasing the insurer vs the best and/or most thorough repair.
Forcing the use of a specific repair shop should not be legal: check with your State's Insurance Commission, who hopefully is consumer-focused vs being an insurer's rubber-stamping back-up.