Originally Posted by GUWonder

Due to airport/gate set-up at most major Schengen zone airports, flying intra-Schengen flights requires clearing Schengen zone immigration; therefore a visa would be required of those persons who otherwise require a visa to enter the Schengen zone.

If it wasn't this way, those who wished to evade border control could just buy a ticket with an intra-Schengen transit and exit on arrival at the second airport in the Schengen zone without clearing customs.