One of the biggest issues, I've heard, is having mesh type designs on the shoe. And it isnt a UPS thing, its an OSHA thing. They claim a shoe with mesh is more likely to rip or tear and cause injuries. Thus being unsafe.
Im wearing a pair of Braham steel toed shoe. I was wearing the same brand, but was gray in color and mesh was easy to see on it. I switched to the black colored ones, still has mesh, they dont say anything about it. Its because it looks solid, a complete shoe with no mesh. Funny, when you first get the paper about working requirements, all it says about shoes, that they should be nonslip, thats all.