[richooret]

You misunderstand the purpose of "Sent from my iPhone" - it was a status symbol, it showed that the sender was part of the superior iPhone owning elite. It was trivial to remove, but most didnt "oh, I am too busy too remove it, I guess I'll just leave it and let everybody know I can afford an iPhone".

You are right, it was advertising, but it advertized the user, not Apple.

[poly2it]

That's advertising with extra steps. Apple having created an ingroup and an outgroup is very effective advertising on their side.

[opello]

I always thought this was an implicit request to forgive obvious typos and autocorrect mistakes. Sent from a mobile device (iPhone, Samsung Galaxy, Blackberry, Windows Phone, etc.) with a tiny keyboard and in a setting in which proofreading may not be as rigorous as normal.