Having "Pottermore" as a tag only makes sense when the question is about Pottermore. As you mention, this applies to two of the (currently) five pottermore-tagged questions:
My best guess is that, for two of the remaining three, it was used because the question was motivated by information gathered from Pottermore:
This is not, to my mind, a good reason for a tag. It's worth pointing out that one of the suggested edit rejection reasons is:
This edit introduces tags that do not help to define the topic of the question. Tags should help to describe what the question is about, not just what it contains.
No sensible interpretation would argue that those two questions are about Pottermore; they're about the Harry Potter canon, and we already have a tag for that.
The final question, Why isn't Draco Malfoy behind bars in Azkaban?, I assume had the tag added as a way to motivate answers - perhaps to indicate that Pottermore is an acceptable source. Again, the question isn't about Pottermore.
If we're going to keep it1, that's the only sensible usage guidance: for questions about the Pottermore website.
1 And, personally, I don't have strong feelings either way; it's clearly not a tag that's getting used a whole lot, but it also doesn't really hurt anyone to have, so long as its use is policed