Yeah, it's a warning that was passed by ballot initiative. If the initiative had said "Warning: this building is known to contain body thetans" then they'd have to print that on the sign. Welcome to California.
To me, these labels are useless and dangerous for the same reason a lot of anti-drug education/propaganda is bad: it doesn't do anything to inform you about the relative level of danger. e.g. regardless of the affects of the drug itself, smoking a joint is _much_ less dangerous than injecting any substance with needles/syringes/etc which are not sterile.