I wouldn't call them an "allegory," since that's a specific literary type (like Pilgrim's Progress.) But, even so, I don't fully agree. Allegories (or literary analogies) are always the fruit of the author's understanding.
Consider the Christ parallels of the Matrix trilogy, as opposed to the Christ parallels of Lord of the Rings. The Matrix was far more Gnostic Christian than orthodox Christian - and that was the result of the Wachowski brother's immersion in eastern mysticism, Christian gnosticism, and Greek philosophy. LOTR, on the other hand, was the result of Tolkien's immersion in the Catholic faith - so elements of the Christian myth(*) were borrowed for his LOTR story.
If you read The Deathly Hallows, you find the same sort of thing. It parallels The Lion, the Witch, and the Wardrobe much more than the Matrix (which surprised me - I predicted a Matrix Revolutions-style ending, where Harry Potter would have to allow himself to embrace death to defeat Voldemort.) The ending Rowling puts in the Deathly Hallows is not the sort of result an unbeliever would choose. It even involves a conversation between Harry Potter and Dumbledore that could have been given by Aslan (and earlier, there are two unambiguous Biblical quotations.)
(I'm constrained by the desire not to give any spoilers - so I hope my point is being communicated through the vagueness.)
(*) When I use the term "Christian myth," I do not mean that Christianity is not true, but instead that it has the epic resonance of a mythical story. I use the term in the same sense that C.S. Lewis did.
I would agree that allegory is a literary type. And if an author has taken the time to write an allegory, the reader should try to understand the author's meaning. But if the author has not provided a key that allows the reader to translate unambiguously from symbol x to definition y , then there will always be a problem of interpretation.
As C.S. Lewis says, there is nothing written by the hand of man that cannot be allegorized. He was often amazed that people read meanings into his works of which he had no conscious idea when he wrote them.
In the afterword to his The Pigrim's Regress, Lewis states
When allegory is at its best, it approaches myth, which must be grasped with the imagination, not with the intellect...It is the sort of thing you cannot learn from definition: you must rather get to know it as you get to know a smell or a taste, the 'atmosphere' of a family or a country town, or the personality of an individual.In his essay "Tolkien's The Lord of the Rings", he states the following:
What shows that we are reading myth, not allegory, is that there are no pointers to a specifically theological, or political, or psychological application. A myth points, for each reader, to the realm he lives in most. It is a master key; use it on what door you like.For a simple allegory, there would be a one-to-one correspondense between symbol and meaning. For the best allegory, and here we enter myth, there can never be a single meaning.
The Harry Potter books may not be allegory, even though they may be allegorized by a clever reader. But I do think Rowling has created stories that approach myth. And as myth, every reader will have to enter upon his own path in that enchanted world she has created.