If the ATF has never been changed, and if the shop has been burned by unscrupulous owners blaming a flush for the trans failure, they have every right to deny doing any services. Its somewhat a free country still.
Problem is that the ignorant unmechanically inclined insurance companies, lawyers, attorney generals, jurors, judges, news outlets, consumer activists...... force settlements against too many shops even when not at fault. Even 'signed' risk waivers have been beaten in the judicial system.
The problem is that many shops 'scrape' by on profit margins these days. The low profit margin vs. risk on poorly maintained vehicles isn't worth it.
A way around this is to tell the shop that your in for your regular 20-40k flush that you do every 20-40k miles every couple years. Tell the shop that they can take a test ride before hand to verify transmission shift quality. Any slippage clunkiness shift quirkiness means issues prior to flush, and will be noted as 'trans failure not repairable by flush' on the receipt. Many shops will overlook mileage if they perceive a history of maintenance. Be willing to sign a risk waiver. Make sure he shop doesn't use any solvents during the flush. If there is a serviceable filter, allow the shop to change it. Either bring your own ATF or demand to know which ATF that they will be using and verify it is acceptable. Also, let the shop know that you are willing to forgo any receipt for the flush and pay cash.