Thank you for your 1st post contribution, but unfortunately you're wrong and I'm right, its not intended and is an exploit
You give such a compelling argument there. Here's mine..
Quoted directly from the spy expertise page, one point in decoy "Creates a decoy that will
draw attacks away from the spy". When used in PvE decoys does a taunt to the mobs attacking the spy, thus drawing attacks away from the spy. To give it a use in pvp, the Devs made decoy that when used in pvp, whoever is targeted on the spy will automatically target the decoy, and all damage will go to the decoy.