Welcome to the Olympics. This kind of thing happens all the time. The Olympics are a time to overcome and persevere no matter what your adversity may be. It is a once in a lifetime chance to empower others through your powering through defeat. I think anything you face in the Olympics is fair game.
Yes, fluctuations in weather is a fact of life in all matters. It is also a slippery slope. If one athlete requests a do-over due to rain, why can't another athlete do the same because it was more hot and humid the day he did his race? How do we compensate for headwinds and tailwinds to make it "fair"?
All runners understand that they're performance is going to be outdoors. They all sign up knowing fully that they may face adverse weather. It is not as though the runners summoned good weather with their special weather machine that they bought because they have more money or anything like that.
Rio has had some wacky weather. If you look at diving, some of the athletes almost got blown off the springboard, or into the springboard, and you could see that they were blown off course. So for those that didn't have the same conditions, it was an advantage. For runners, the rain can make a big difference if the track surface is slippery or visibility is really poor, and in those cases, the affected runners should get a "do-over" as long as it doesn't hurt the other competitors who fairly advanced.