It matter little when work ouside the classroom, is done, or by whom; the thing is that the system should allow sufficient resource for that which needs to be done. If it does not, it is failing.
And that has little to do with the initial question which was about why the education system thinks it can take days out of the normal school schedule when it opts not to teach the children it is supposed to be teaching. Never happened when I was at school, no excuse for it now.
If there is no need to have the teacher away from the classroom, then there should be no training days. If a need can be shown for teachers to have more time away from actual teaching to cover other tasks that they can not fit into the agreed normal working hours, then a supply teacher should be taking over the teaching.
It is pointless to squabble about whether it is teacher's fault or the parents', or whether this away from the classroom work is necessary or superfluous, the system is clearly failing one way or another, and ought not be defended.