¿Por qué puede el estado prohibir conducir sin cinturón de seguridad, meter droga, suicidarse o abortar?

A diferencia de las religiones a sus fieles, el Estado no puede prohibir nada a sus ciudadanos. Si mucho, puede imponer sanciones a quienes incumplan una ley. Ya sabiendo las consecuencias, cada quien verá si acata una norma porque tiene sentido o por temor a la sanción, o como en la mayoría de los casos, no la acata confiado en que no lo van a pillar.

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Why can the State forbid to drive without a safety belt, take drugs , commit suicide or have an abortion?

Unlike religions to their churchgoers, State cannot forbid anything to its citizens. At most it can punish those who disobey a law. Knowing the consequences, people is free to follow the rules because they make sense or because they are afraid of being punished, or, as in most cases, they don't follow the rules because they are confident of not being caught.

Ask me anything. I deal better with "why" and "how" than with "what".