I had a sponsors that used to financially encourage me to compete in certain events sponsored by companies that were their rivals, and loved it when I could do well in those events. Of course they encouraged me even more to compete in events they sponsored, and loved it more when I could do well in those
It is sort of like when you go to another school and beat them during their homecoming event.
I seriously doubt Chicago was rejecting any entries from East Africans (or anyone else) sponsored by adidas.
More likely is that money for appearance fees, travel and other invited elite athletes was limited this year, and they concentrated what they had on Nike sponsored athletes.
Nothing would have prevented any adidas sponsored athlete from paying the entry fee and running the race.
But with the way marathons are organized now, they may not have been able to get into the elite field corral/wave to start separately from the rest of the field. So you just about have to be an invited runner to be eligible for prize money in many races.
Many pro running races are now separate events. open only to a limited invited number of athletes. Running is not like it used to be, when