Sorry but, because of how artists really make their money (touring) yes they are last minute(ish) decisions. No major act that is about to go out on the road is going to build their tour around the PanAm games (or even the Olympics)....the exercise is the opposite it is "ok, we are close enough now that tour dates are set...let's see who the biggest/most popular names are that are available and we can get".
All of this armchair criticism of the entertainment selection is kinda funny.....because not only did the organizers realize that this was something best handled by professionals....they went out and outsourced it to the world's premier tour/booking/promoting company. Live Nation is kinda good at this stuff.
Now, I only used Rush as an example of tour conflicts because they were mentioned and I happened to be in Vegas this past weekend.....did not mean this to be about Rush....but let's ask ourselves......assuming (wrongly IMO) that tour dates could be built around the ceremony....what is the average age of Pan Am athletes (since closings are supposed to be their party)?.....ok, assuming you got the right answer, ask yourselves...how many of them would rather see Rush than Kanye/Pitbull? I know that I would rather see Rush.....but the show is not designed for me......nor should it be.