It all depends on what's in the contract for the appearance fee. If the contract requires appearing but not finishing, then that's what it requires. If it requires appearing and finishing, then it requires appearing and finishing. Each company is going to have a slightly different way they set these up and it will likely vary athlete to athlete.
Mostly, they want to draw your attention to the race. Once they accomplished that by having their star line up, they've accomplished that goal.
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