AKA, the grid girls and walk-on girls are there for the purpose of objectification. The other mentioned are there to serve a purpose, with any objectification being secondary at best. You could also argue that the former are told what to dress by the event organiser, whilst the latter are in control of their own wardrobe (or their employees are). I'm unconvinced by that argument personally, but it certainly gets made. FTR, the same argument gets made about any "slutty" uniforms dictated by employers.