Avodah Zarah 56 - Earlier and Later Rulings
Sometimes the Sages of the time of the Mishnah (initial rulings issued before the time of the Talmud) changed their opinion, and such is the case here . Later rulings were stricter with the wine and more lenient with ritual purity. Thus, now the grape juice was forbidden immediately once it began to flow freely out of the grapes, but on the other hand one was allowed to harvest grapes together with an idolater, and there was no concern that the grapes would later become impure.
However, this leniency applied only to an idolater, because for him there was no requirement of tithes. A Jew was supposed to keep his grapes pure, since later he had to give a portion to a Kohen. Consequently, one was not allowed to harvest grapes together with a Jew if that Jew was going to make them impure - because one should not help people who transgress.
Art: Edwin Deakin - Grapes