What's wrong with parallel conductors?
Both configurations will conduct power to the loads.

When trying to figure out what's 'illegal', and why, you need to understand what fault conditions the authorities are trying to prevent. There may be a commentary in the relevant standards if you're lucky.

In the UK, such an arrangement of a circular conductor is called a 'Ring Main', and was actively promoted for domestic rewiring from the late 1940s onwards, due to a shortage of copper and high rates of house building following World War II. Having two paths back to the distribution box allows lighter conductors to serve the same area than could be served by spurs.

The rules are that a 2.5mm2 conductor serves an area of up to 1000 sq ft, and has both ends returned to the distribution panel, protected by a 30A fuse. Each socket on the wall that is part of the ring has a loop in and loop out of 2.5mm2, connected at the socket terminals. Note that a spur of 2.5mm2 would use a 22A fuse.

The problem comes if someone replaces a socket without putting both conductors into the terminals, or a conductor breaks somehow. The loop is now broken, and we now have two 2.5mm2 spurs, needing a 22A fuse for protection, but having a 30A fuse, with no apparent failure to alert anybody. 

Any paralleling of wires allows this sort of undetectable potential overload error to occur. Some regulatory authorities ban the practice, some permit it.