Parents "lie" to their kids about things like Santa because they remember the magical world that was created for them from their own childhoods, before the reality of a world with no Santa, murder, disease, death etc intruded on their reality.
Which is better, telling your child:
"yeah you will lose all your baby teeth and each one will hurt and bleed and make you look silly with big gaps in your mouth"
or
"oooh you lost a tooth, now let's put it under your pillow and the tooth fairy will exchange it for a little gift while you sleep"
I don't understand why you put "lie" in quotes. Even if you believe it is done for a good reason, it is most certainly lieing.
You have a wide variety of choices as to what to tell your child that doesn't include lieing. You can celebrate the loss of a tooth as a positive step of growing up without telling a lie about a creepy fairy that sneeks into houses and collects teeth.
Adults may rationalize to themselves that they lie to children to protect the children, but I think it is often more motivated by the adults' desire to avoid having an uncomfortable and difficult conversation.
Which is better, telling your child:
"yeah you will lose all your baby teeth and each one will hurt and bleed and make you look silly with big gaps in your mouth"
or
"oooh you lost a tooth, now let's put it under your pillow and the tooth fairy will exchange it for a little gift while you sleep"