SAN FRANCISCO – In a lighted garage on one of San Francisco’s busiest streets, a young man in baggy trousers and messy brown hair pulled down his pants. He had been hiding two pairs of stolen jeans with the tags still on them. He handed them to another man waiting nearby, took some money, pulled up his pants and headed back into another store on Market Street — home to the city’s high-end designers and big-chain retail shops.The incident wasn’t a one-off. These brazen acts of petty theft and shoplifting are a dangerous and all-too-common consequence of Proposition 47, a referendum passed five years ago that critics say effectively gives shoplifters and addicts the green light to commit crimes as long as the merchandise they steal or the drugs they take are less than $950 in value. The decision to downgrade theft of property valued below the arbitrary figure from felony to misdemeanor, together with selective enforcement that focuses on more “serious” crimes, has resulted in thieves knowing they can brazenly shoplift and merchants knowing the police will not respond to their complaints, say critics.