The reason is that Reb Moshe allows cholov stam because the government watches out for mixing milk from a non-kosher animal so there is mirthas, fear involved. The question they put up is if the chachomim forbid milk from a non-jew with a count or we are doing it from custom which makes the prohibition biblical because of a neder. Certainly if someone can get cholov yisroel then he should rather get it.
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