magic “MENU” button under Twenty Twelve theme

This is the intended behaviour on an iPhone or other mobile device (it’s called ‘responsive design’).

Because you have a much narrower screen on a mobile it’s very likely that if you have more than four or five top-level pages, the navigation menu starts to wrap on to two lines and become messy & difficult to use.

There’s no simple disable option, apart from writing a child theme that overrides the styles.