ST tires are made with a compound that doesn't dry out as fast when it's sitting, and the sidewalls may be a little stiffer.
As long as the tire has a load rating high enough for the load, meets minimum tread depth, and is properly inflated, with no cords exposed, what can they say?
There are all kinds of semi trailers running around with under-spec steer tires (tires on a steer axle have a deeper minimum tread depth than drive or trailer tires per DOT), drive tires that lost their mates, and recapped steer and drive tires out there. Are they going to take all those out of service? I don't think so.