independence of events $A$ and $B$

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In this proof on Wikipedia of the Schwartz-Zippel lemma, are the events $A$ and $B$ independent? My reasoning is that $s_1$ is chosen independently of $s_2,\cdots, s_m.$

If they are not independent, then why is it correct to claim that $P(A | B^c) \leq \frac{i}{|S|}$