Possible problem with busybox sh handling of pipes

gistfile1.txt

/tmp $ cat spawnShortPipeSleep.sh 
#!/bin/sh

sleep 5 | logger -t ZZZZZ &

Running this under bash:
/tmp $ bash --version
GNU bash, version 4.1.5(1)-release (x86_64-pc-linux-gnu)

Running this under bash returns immediately:
/tmp $ bash -c "./spawnShortPipeSleep.sh"
***Returns immediately***

Piping the output to another process, returns immediately :
/tmp $ bash -c "./spawnShortPipeSleep.sh | logger -t SPAWN"
***Returns immediately***

Running this under busybox:
root@navetas ~$ sh --version
BusyBox v1.15.0 () built-in shell (ash)
Enter 'help' for a list of built-in commands.

Running this under sh returns immediately:
root@navetas ~$ sh -c "./spawnShortPipeSleep.sh"
***Returns immediately***

Piping the output to another process, returns after 5s :
root@navetas ~$ sh -c "./spawnShortPipeSleep.sh | logger -t SPAWN"
*** Returns when the 5s sleep finishes! ***