Any tensioning of flying wires has to be carried out at the pole (officially).
As an aside, the wires aren't meant to be bar-tight, there is supposed to be an element of slack in them for the very reason you state above (wind). I suppose it's what one classes as 'too slack' .
That explains what they said to me. Its an odd decision given I could see mine had pulled away from the house side so tensioning from the pole would have resulted in no slack at the wall any more which surely is bad?
That said I can see why for most properties it makes sense. I'm an outlier in that they can easily go round the side of the house for access, even if we weren't home (we always are mind you). Many properties wont have that luxury.
I still haven't remember to look to see what mine looks like now, I suspect like you said they will have tightening it from the pole.