If it is really made out of stainless steel, I don't see any problem putting it in the oven. A lot of things can be put in the oven: stainless steel, regular carbon steel, aluminum, even Teflon.....
Maybe the stainless steel cooling racks have some plastic feet or something. I am not sure.
I have stainless steel racks for both half and quarter sheet pans. Have never had a problem with them, and use them all the time. Probably just some attorney advising the seller that someone, somewhere, brought a pseudo lawsuit about being harmed by stainless steel racks. My advice, ignore the comment.