WPS isn't meant to be 'on' indefinitely. Pushing the button provides a short 3 minute window where you can push a button on other devices and they can quickly join your network without typing in all the security settings etc.
WPS Push button method:
In which the user has to push a button, either an actual or virtual one, on both the access point and the new wireless client device. On most devices, this discovery mode turns itself off as soon as a connection is established or after a delay (typically 2 minutes or less), whichever comes first, thereby minimizing its vulnerability