If the boots are old and the heels and toes have been worn down, the boot will not fit well in the binding. The shops check for release points and don't want to be held liable for personal injury damages if the binding - boot interface does not work to their satisfaction. The problem is easily solved, by signing a waiver on the mounting and binding testing. Even then , if shop can't get the boot to not wobble in the binding after binding adjustment they won't motnt the binding with your boot. To get them to mount it, you can get the size of the your boot in mm or mondo size, and tell them to mount the binding using a boot they have in the shop with the same mm length as your current ski boot. . I beleive the heel and toe pieces height length, etc of ski boots from boot manufacturer to boot manufacturer is standardized.
One Caveat - I would check to see if the ski boot wobbles, or the binding setting is turned up way to high to surpress the wobble. The shop is concerned about your safety if this is the case. Easy to see if your boot is damaged, - set the boot on a flat surface and see if it stands solidly upright with no wobble. Give it a slight sideways push, not enough to knock it over, but with just a slight touch to see if rocks from side to side. If that is the case, a new pair of boots is in order. They are probably causing alignment issues if they wobble. You will enjoy the sport more, if you don't have these alignment problems caused by your boot.
Another issue, is that shops are reluctant to mount older bindings that more 7-10 yrs old for the same liability reasons, and mounting skis is not a big money -maker for them unless they sold you skis. So the money made in mounting is not worth the liability risk involved.