Young Justice co-producer Brandon Vietti has talked about the Justice League’s Watchtower on his blog. He describes the logic that went into the design and its in-universe origin.
On Earth 16 the Watchtower is a secret place so I couldn’t design it in a way that might imply the Justice League hauled up loads and loads of building materials from the planet and put the thing together. There are too many eyes on the League. They’re famous. There’s no way they could build a space station without someone finding out about it.
In order to acquire some orbital real estate that met some very specific requirements I figured the League would have to turn to their most trusted outer space connections, the Green Lanterns. This led to the idea of the Earth 16 Watchtower being a decommissioned Green Lantern base station. It was towed to Earth by Hal Jordan and John Stewart.
Go see Brandon’s blog for more details and pictures.