I think the question is whether being an IAAF or USATF race means that you can't allow people banned by WADA to compete. Chicago is an IAAF Gold Label race/Marathon major and the course is certified by the USATF so it must fall under their rules in some way.
I see that the IAAF rules require doping controls-
I don't know if these rules apply to everyone or just elites.
Anyway, if the IAAF or USATF require the ban, then Chicago didn't have a choice.