I was asked by my teacher to look at Tosfos to anwer the question:
Is this case(the candle)similar to the Mishna on 121a-telling a nachri to extinguish a fire?
AND
How can any case be asur, if we just learned on 121a the a nachri always acts in his own self intrest?
Tosfos writes that we only rely on the fact that what a Nachri does is for his own interest where the Yisrael does not proceed to do something that takes advantage of the act.
When the non-Jew put out the fire, it is Mutar. However, when he lights a candle and now the Jew is going to USE the light of the candle, or when the non-Jew cooked and now the Yisrael is going to EAT what he had cooked, we do not rely on the reasoning that the non-Jew did it for his own self-interest, to permit it.
Dov Zupnik