Shadow Weaving

I have noticed when playing my Shadow priest alt that Scream will trigger Shadow Weaving.

According to the tool tip it says “Your Shadow damage spells have a 100% chance to cause your target to be vulnerable to Shadow damage. This vulnerability increases the Shadow damage dealt to your target by 3% and lasts 15 sec. Stacks up to 5 times.”

I was under the impression its only spriest damage spells, not all shadow school offensive spells like scream, but ill take it. (Maybe intended?)