A micro switch doesn't have a GND either. It has a COMMON though.
The pins are: Common, NO (Normally Open), NC (Normally Closed).
In this case, Normally means that the switch is in rest situation (NOT pushed).
The Common pin is connected to the moving part in the switch. So when the button is not pressed, the contact will be made to the NC pin. When the button is pressed, the moving part will switch from the NC pin to the NO pin. That's why you always hook up a microswitch between Common and NO (in the case of arcade pushbuttons).
A leaf switch simply only has a common and a NO and you can choose which is which. Must be one of the most basic concepts in electronics