QUESTION: Why are some people strict with respect to chalav Yisroel milk and lenient with regards to “chalav stam” butter?
ANSWER: Shulchan Aruch (YD 115:3) writes that it is permissible to eat butter made from chalav akum provided it is cooked to the extent that the liquid milk portion has evaporated, and there is no local custom against consuming this product.
The Shach (YD 115:27) explains this is so because non-kosher milk cannot be churned into butter. Since it is obvious that butter only contains kosher milk, the restriction of chalav akum does not apply.
Rav Belsky, zt”l noted that nowadays butter may contain flavor additives such as starter distillate which could be made from regular milk (chalav stam), and, therefore, one who does not drink chalav stam should not purchase this butter. One should not assume that butter today has the same status as the time of the Shulchan Aruch.