I have to agree that it has more to do with intention here. If there is a ghost already present no one could blame a magic practitioner for working with them but to call up a spirit and force them to stay seems to be what this law was more about. If having contact with the dead was out of line then the guy who spoke to the dead that Harry got his information from in Dead Beat (I don't remember his name and I don't have the books handy) would be in trouble all the time. He also calls the dead but they come if they are willing - I didn't get the impression that he forces them back to answer questions.
Although I don't know how to fit this law in with the Bob from the TV series other than to say the White council seems to allow the breaking of the laws under curtain curcumstances for themselves alone.
Ectomancer's name = Mortimer Lindquist
There are also several hints at there being several "levels" of Ghost
- the rarest being one that has sentience.
More than just an echo of a specific memory, or emotion.
Morty talks about the ghosts as if there were "real" people.
And Harry's dad showing up [Dead Beat] - and acting like he would have if he was actually alive.
I think that Morty's warning from a Warden was to not try the "Enslave a Ghost" side of Necromancy.
Talking to them, asking questions - or for their cooperation, is alright.
Though I tend to agree that if Harry had displayed the same abilities as Morty,
Old Morgan would have tried to come down on him for breaking the Law.
New Morgan would just have him watched a lot closer, and not say anything to Harry.
Remember that the Laws apply on in regards to Mortals.
Bob is an Air Spirit from the NeverNever, residing in a skull. He was never Human.
For the Laws that apply to Supernatural Beings, see the Unselee Accords.