R Moshe's psak was specifically that there is a greater fear of the law in the USA.
R Moshe's psak wasn't because of the law, it was because of the level of enforcement and the public fear of the law.
Reb Moshe does constantly use the words חלב הקאמפאניעס
במדינתינו, but I do not recall him explicitly saying that “there is a greater fear of the law in the USA” than elsewhere. I would be glad if you could provide a source/quote.
Also, Chelkas Yaakov argues that this is a reason to be Machmir, since someone who is accustomed to be Meikal in USA will not realize that he needs to be Machmir when travelling. A similar sentiment was already expressed by the Tzemach Tzedek, albeit not with regards to the USA, obviously.
Even in the USA, Reb Moshe emphatically prohibited milk purchased directly from the dairy farmers, and only allowed it to be purchased from the companies. How many people know that?
Either way your story is not relevant, as halacha does not require absolute knowledge.
Your story is also not fully relevant since in halacha we can rely on things that are not absolute proof, roiv, eidem, chazaka, etc. So the fact that it's technically possible for the milk to be from a pig is irrelevant. As a matter of fact the issur of cholov stam is not a regular safek that we are afraid it comes from a trief animal, there was a special takana to be chosheish.
You words are disingenuous. The איסור in ש"ס ופוסקים is known as חלב שחלבו עכו"ם ואין ישראל
רואהו.
Most Achronim accept that Chazal mandated a higher standard than elsewhere, and there has to be actual seeing, or something on that level. [Even Reb Moshe holds of this to some degree.] The fact that “in halacha we can rely on things that are not absolute proof, roiv, eidem, chazaka” is not applicable to Chalav Yisroel, because of this higher standard mandated by Chazal. Sure, you can claim that Reb Moshe (and his followers / successors) were Meikel in this regard, but don’t pretend that
every other Poisek doesn't exist.
R Moshe's psak wasn't because of the law, it was because of the level of enforcement and the public fear of the law.
Does that still exist today?
Mishneh Halachos claims that it doesn’t. [One interesting point he makes is that no self-respecting Frum person would rely on “Kosher Laws” to accept that the meat a butcher was selling is Kosher.]
A while ago, there was circulating an email exchange between Rabbi Aron Teitelbaum (?) and some State and Federal agencies, who went on record saying that there is no real government oversight in this regard.