No Some adults can't. I still get emails from people thinking they will get Gap certificates for forwarding an email.
The problem is that the marketing behind the books push them at our kids and SOME people don't like the concepts in the books. Kinda like the whole Captain Planet push that corporations were bad and environmentalism is good. I don't want that stuff pushed at my kid. And I don't want them ridiculed because they don't go along with it.
Probably the way many atheist parents feel about prayer in schools.
These are "fantasy" books. If your kids wants to read Harry Potter, and they, for whatever reason(?), cannot tell that the books are a complete work of fiction, then I suppose it is your job as a parent to tell them otherwise.
As to adults who can't tell the difference, I'm sure if you ask those same adults who think they'll get GAP gift certificates, if magical brooms and gnomes are real, and I'll bet you'll get a resounding "NO!".
I don't want to ever see a kid ridiculed for anything. Being a kid is though enough. I am sorry if this is happening to your kid.