Let's also consider the trailer manufacturers recco's. I have two Titan horse trailers and a Load Max equipment trailer in addition to my travel trailer. The owners manuals for both Titan trailers and Load Max state that either ST or LT is correct fitment for their trailers.
Seems to me they are giving the consumer the option to either save some money by using ST or use LT if they see fit.
Some travel trailer companies fit LT either as standard or extra cost option. Airstream is one that fits LT as standard on some of their heavier models, and optional on all models. Oliver fits LT as standard on all their models. I suppose their are others as well.
