@warrenchmobile Explained it best; a non sentient object you placed that can determine neither friend nor foe and merely does what is does: shoot stuff.

And I have to call you on your counter argument @Smetrix because the ballista (which is static) even though an active defense versus a passive one, cannot identify friend vs. foe. It is non sentient. Active vs. passive does not inherently mean the active defense can differentiate.

Is it annoying to be targeted by your own 'stuff'? Yes.

Is there a reason for it? I can see a few scenarios in which ballista could be used to cheez some mobs, but ultimately the real reason can only be revealed by the developers.

So for now we can deal with it or not build them.