So the question of "who you gonna call?" Is answered with a shrug?
It seems really odd that you wouldn't have need of something like that.
Every argument and situation that went into the creation of Grid Overwatch seems to have a magical parallel. Of note is the Astral/Matrix cross jurisdictional problem. You move at what, 6000kph when you're astral? If a guy who's meat body is in the CAS goes astral and heads over to Seattle to summon up some spirits on Aztech property to commit a crime in a Renraku facility who deals with it?
Sure individual police companies have their own magical investigation groups, but then again they also have their own cyber-crime specialists and hackers. Yet Grid Overwatch is still a thing.
Magic is freedom, all you need is your magic, your bike, and the open road.
And we all know how the Corporations feel about freedom. I'd also question the implied statement that the matrix can be easily controlled just because it's a creation of man. If that were true then why have so many organizations out policing it and yet still things are still so wild west. In addition there's things like AI's and sprites and ghosts in the machine. Sure individual nodes and components can be controlled, but the trix as a whole? In theory sure. In reality, good fragging luck chummer. I understand what you're saying, and to a degree I agree that it's a matter of degree, but just because it's easy doesn't mean it's not something that would be attempted.
The parallel is there with magic as boomstick indicated. Sure you can't control magic as a whole, but you can control individual mages and entities and localized regions.
"Reality" of the situation aside, don't you think the public at large would
demand some sort of Magical overwatch division? Think about their perception of things magical.