Inline Barrel Adjusters. Why are they so badly designed?

[PpPete]

OK, so these rely on being able to turn one part with respect to the other.
Why the hell is only one half knurled?
As soon as there is a bit of tension in the cable the black part just spins around as the red part is turned.

 

[Webbo2]

So if it’s for your front mech. Put the chain on the big ring click you lever to drop it in to little ring. Don’t move the chain so its still on the big ring but now with a slack cable. Now it’s easy to tension.