There is a lot more to fit a tire on a wheel then the measured diameter, and tito has the right idea in thinking about width.
The liability comes in because it isn't a stock tire size or recommended replacement size. Some shops will only install stock sizes because should something go wrong with the vehicle down the road, they don't want the problems being blamed on them and then you try to get the shop to pay for repairs and/or damages. Most of the time this seems to happen with "chain" type shops. Find a more independent retailer that deals with custom cars and trucks, they'll have no problems mounting the tires for you, and should something happen down the road, they'll be glad to fix it for ya for a fair rate.