According to the Magen Avraham and Mishnah Berurah, the vats that are used to pasteurize chillul-Shabbos milk become treif. Therefore, even milk produced during the week in these vats should be forbidden. Rabbi Zoldan (Nesiv Hachalav, p. 84, published by Tenuvah), however, justifies the use of these vats by explaining that even according to the Magen Avraham, food cooked on Shabbos is considered treif only to the person who cooked it. Although the Ksav Sofer also forbade others from eating food cooked by hotel owners on Shabbos, he did so only to prevent the proprietor from continuing his folly. He did not actually consider the food to be treif to such an extent that it would cause the pots to become forbidden to those who did not cook the food.
Furthermore, by purchasing chillul-Shabbos milk, one encourages the producers to continue milking their cows on Shabbos. However, purchasing weekday-milk that absorbed the smallest residue of chillul-Shabbos milk in no way, presumably, encourages them to continue milking their cows on Shabbos.