Rabbi Hershel Schachter
About fifty years ago the Yiddish press carried a news item that the Vaad Halacha of the conservative movement issued a "psak halacha" that one may drink Welch's Grape Juice. Their reasoning was that Talmud states that there is no prohibition of stam yainom on yayin mevushal and the grape juice was cooked.
Rav Soloveitchick came into his class the next day, related to the students what he had read, and asked if anyone knows what was incorrect with the statement. The only one among the students who knew anything about the topic at the time was Rabbi Aharon Lichtenstein who had a smile on his face. The Rov asked him to explain to the other students where the error was. So R' Aharon explained:
The main reason Chazal prohibited stam yainom was out of fear that it could possibly lead to intermarriage; the concern that perhaps the nochri may have been menasech the wine and then later allow someone to drink it was very farfetched. However, once Chazal instituted the prohibition out of concern of chasnus, they extended the issur to include even kosher wine handled by a nochri lest the nochri was menasech it for avodah zora. In the event that the wine had previously been cooked, it would be even more unlikely that the nochri would be menasech it, and therefore in that case magah ha'nochri would not make the wine prohibited. But since in the case of Welch's Grape Juice the wine was processed by nochrim before being cooked, the fact that they cooked it afterwards was irrelevant. The wine was forbidden because the concern of b'noseihem (intermarriage), which is the primary reason for the issur of stam yainom to begin with, still applied even though the farfetched concern of nissuch no longer applied.
The fatty parts of the sacrifices that have to be burnt on the mizbeach must be raw; if they are first cooked, the kohein does not fulfill his mitzvah of haktorah. This haktorah lacks the element of raiach nichoach because the smell will simply not be the same. Similarly, the blood of a korban may not be cooked before being sprinkled upon the mizbeach; if it is cooked first, it's not considered dam (blood) but merely the "juice of the meat". It is for this reason we assume in Shulchan Aruch that eating dam shebishlo is only forbidden m'dirabbonon - such blood would not be acceptable in a korban, and that is the entire basis for the biblical prohibition forbidding dam.
The same is true regarding wine. Yayin mevushal is considered inferior and would not be accepted for nisuch on the mizbeach. Since it would not be accepted on the mizbeach in the Beis Hamikdash, we assume that the nochrim would probably also not use it for their avodah zora. For that reason, if a nochri handled kosher wine where there is no issue of "binoseihem" but only the concern of nissuch, if the kosher wine had already been mevushal the chachomim never prohibited it.