The following answer is my opinion based on my experience as a mashgiach in 3 different settings: a) meat restaurant, b) Pesach at a hotel, c) at a Kosher supermarket, as well as some common sense [being a business person, myself] - if a person assumes that all is well with a product just because it has a hashgachah, then that person is being naive. Granted, it is probably better than assuming such of a product that does not have hashgachah. Yet, there have been plenty of instances where a specific "kosher" supervision either proved to be a sham [in Monsey, several years ago in the kosher meat scandal; as well as one that happened in Florida over a decade ago] or breaking laws related to [if I understand it correctly] price gouging [and are, consistently, fined by the government for breaking the law] as in what happens with at least 1 certain unnamed company during Pesach [yearly].
I have seen it reported that the reason why cholov yisrael milk [and in some milk products] goes bad faster than cholov stam is because of shipping and storage malpractice.
bottom line: the halacha is that you can't drink milk from an unkosher animal. the point of Cholov Yisrael is to ensure that the milk is not from an unkosher animal. Yet, if you are familiar with the kashrut supervision at companies in areas where Jews do not live, or at least are not in close proximity, around the clock, on site supervision is not guaranteed. Thus, how do you know that the company doesn't sneak something unkosher into their product when the mashgiach is not at the plant, in the same way you question the possibility that a company doesn't sneak non-kosher milk into their product when the government isn't there?
If a company is willing to be, potentially, caught for violating a government food law that will cost it its right to either operate or produce a certain profitable product, that is foolish business practice.
A final related note: in the Shulchan Aruch it says that, when it comes to shechitah, what matters is that it is done according to the halachah, not the shochet's yirat shamayim. This concept would seem to apply here as well.
PS As a note of clarification, brought to my attention by mevaqesh...my underlying point is that I believe that kashrut certification is designed to make it easier for a Jew to purchase Kosher food, and thus it is appropriate for cholov stam to have a heksher, even if it is not necessary in the USA in the present time.