One sun-drenched August morning, armed officers wearing sunglasses and bullet-proof vests descended on a market in Venice, Calif., searching for illegally sold goods. It marked the end of a year-long investigation where undercover agents posed as customers. Their target: raw, unpasteurized milk. Federal regulators say it’s a dangerous and unnecessary public threat, pointing to 143 cases of contamination linked to still births, miscarriages and kidney failure since 1987, the latest involving five California children. Grassroots, back-to-nature consumers say the product strengthens the immune system by keeping intact good bacteria that’s killed in pasteurized milk. The choice should be theirs, the activists say.