Loki muzzled?

I think it's a mix between a nod at the Norse Myths, and a way to keeping Loki under-control. He does have a "silver tongue" (as mentioned in Thor), so por keeping his mouth shut he can't talk his way out of punishment.

I would say it's also to stop him from doing magic, but he's never needed to say anything to use magic before.

because he's simply spitting evil!

I think he was muzzled so he couldn't cast any spells or it was simply a restraining method