Very. That would indicate a level of Absolute control over Magic Itself that would be terrifying. Especially if it's been used to intentionally out Wizards (to their DOOOOOM), and to Limit their Choices and Free Will, which is the fundamental hallmark of the Good Guys. If the Gods themselves want Mortals to Fall, then they are all just plain screwed, full stop.
It doesn't seem any more terrifying than Mab extending winter through her mere presence on Earth. The Sidhe have a vast level of control over daily human lives. And that control would seem to be based on aspects of reality being associated with them.
If the aspect of magic is associated with Hecate (and other mantles possessed by the Sidhe), then it seems logical that they could exert just as much control on that as they can the weather. Or, at least as much control as they have, given their level of influence over the aspect.
All in all, we're not talking about the Sidhe having enough control to cut magic off completely. We're just talking about Mother Winter having enough control (and pettiness) to push out side effects to magic use.
It's just too convenient that wizards are hampered with such limitations without there being an inciting factor for them. If wizards still suffered from boils and spoiled milk, I'd say it's no big deal; the Murphyonic field was always in place, and they only just realized it in the modern age.
But since it's
changed, and in a potentially purposeful manner that successfully alienates them from the rest of mortal society, I say nay, it's not natural.