Fundamentalist Christians believe that Harry Potter is based on Witchcraft. The bible says not to be involved in this kind of thing and so they don't involve themselves or their children.
Someone will ask where in the bible.....there are a few places but this is probably the best known.
Galatians 5:19-23 The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God. But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law.