How is sponsored magic out of their jurisdiction?
For one, the reason Taran describes. A sponsor could simply shield your mind from corruption.
But when it comes to the Wardens, they would not only have to deal with you, but with your sponsor as well. Worst case, this would start trouble in the politics of the supernatural community. They could go through the channels of the unseelie accords, but if it's not a mortal wizard, they can't claim jurisdiction over them. The supernatural world is big on stuff like this.
In fact, in some cases, you could interpret a wielder of sponsored magic as the sword of the sponsor. He is no more liable for his actions than a sword would be, at least where supernatural politics are concerned.
But this doesn't mean they are immune to it all. They could well take a lawbreaker stunt without the Wardens being able to touch them.