Q11. "Can tabloids be restricted constitutionally? I don't mean censor, but can government label tabloids as they label records with explicit lyrics?"
A. First, it should be clarified that the government does not rate movies, nor does it rate music, television, or video games. All of these media are rated by their industries themselves. Movies are rated by the MPAA, and theaters follow the MPAA rules because it is their industry (in other words, there is no federal law that defines what an "R" rating is, nor that tells theaters how to handle R-rated movies). Likewise, the RIAA and record companies rate music. Each broadcast or cable network rates its own television shows, and the TVPGMB monitors the ratings. Video games are rated by the ESRB. Though the government has mandated that TV's contain V-Chips to allow parents to control access to rated programming, the rating is still done by a non-governmental agency.
With that having been said, it really does not matter if the government rates movies, music, video games, and TV or not. The press is a special case, getting specific mention in the Constitution, and its freedom to publish has long been upheld. A Warning Label? Maybe, but it might have the opposite desired effect — do you choose R-rated movies, generally, over PG-rated ones?