Maybe this is the same as baseball or maybe not. In baseball, everyone knows (or should know) that you can be hit by a ball or broken bat on any given play. You assume this risk by going to a baseball game.
Whether this photographer knew (or a photographer should know) that implements can leave the sector in field events is another question. I'm always surprised at how casual people seem to be about field events, but implements leaving the sector are much less common than foul balls or even broken bats going into the stands at a baseball game.
Also, in baseball everyone is presumably watching the play. This doesn't apply to the photographer at EWU who was actually watching the hammer throw, but at a track meet, it can be a 3 ring circus where you have multiple things going on at any given time and you can't expect everyone to be watching every throw. I forget which meet it was, but there was a meet recently where a discus went under the curb and onto the track in the middle of a race just after the runners passed. If the discus had hit one of the runners in the ankle, there's no way you could say the runners should have been watching the discus while running a race. The assumption of risk argument would be much harder in situations like that.
Anyhow, I wouldn't assume that a court would apply the "baseball rule" to every situation at a track meet.