I think all or most tire stores are watching more about what tire/size that came on the item that the tires are being put on and not wanting to stray from that because of the liability that is involved.
That would or could be a nasty lawsuit for both the trailer owner and the tire store if an LT tire blew out on a trailer that was suppose to only have ST tires and caused an accident causing a death. I'm betting the courts would rule in the victims favor that ST tires should have been on that trailer.
Just my .02 and something I always try to keep in mind.