The dementor was not a rogue dark creature searching for souls to devour, nor was it an agent of evil sent by death eaters in order to dispose of one Harry Potter, nor was it an emissary of wizard bureaucracy sent to help discredit the boy-who-lived in a complex scheme, as would be theorized by people in the future and alternate dimensions and time lines.
No, this dementor was from a tiny American town in the middle of the desert, and dementors, no, hooded figures, from that little town were used to doing whatever they wanted, even explicitly odd things like kidnapping interesting wizard children.