Most of the people who criticize the Potter books don't seem to be able to draw any distinction between genuinely sinful witchcraft (namely, making a deal with a demonic spirit in return for earthly powers) and simple magic (which is supposed to work following its own "natural laws" as science does).
The reason for the confusion is that starting a few centuries ago, it was realized that "magic doesn't work" and only modern science (chemistry not alchemy) is capable of being developed into a technology. Many Christians then drew the conclusion that anything that APPEARED to be magic was demonic, because "natural magic" was ineffective. (This conclusion also depended on the assumption that non-demonic (that is, angelic) spirits would not grant miracles, and that God would only do so by His own free choice, uncompelled by our entreaties or offered bargains).
It was a short step from there to accusing people who attempted to do spells of trafficking with the devil (the short step was the unjustified assumption that the spell-casters KNEW that natural magic didn't work and therefore knew that any effectiveness in their magic was due to demons). But the magic in the Harry Potter series is always the natural kind that works according to universal laws and does not involve the intervention of demons.
Can you imagine the fears that natural healers had in those days. By saving someone else's life, you could condemn your own. What a horrible black mark on the name "Christian".
Thank God we don't do that anymore.