Leístas and Loístas
The Spanish-speaking world is divided into two camps: leístas and loístas. The leístas hail from Spain -- not Andalucía or the southern regions, but roughly from the central plateau northward. The southern and southwestern region of Spain are, historically, loístas. Since most of the conquistadores came from southern regions -- many from Extremadura, Latin America is overwhelmingly loísta.
For those readers who have no idea what I am referring to, according to the Royal Academy of the Language, lo is the third-person, masculine singular, direct object pronoun - used for it, him or you, provided that it and you are masculine. According to the same august body, le is the third-person, singular indirect object for both masculine and feminine.
But not everyone has always listened to the Royal Academy... and of course, Spanish was well established as a modern language by the time it was organized in the 1720s. By that time, many speakers, from the geographical areas mentioned above, were already using le as a direct object pronoun --but only for the masculine -- and the usage remains so to this day. The plurals are likewise impacted, so that it is common to hear or read Les vi el otro día (I saw them the other day -- referring not merely to people, but only to a group of men) instead of (as the Royal Academy advises) Los vi el otro día.
Le and les are never used by leístas to refer to a human female direct object or objects. Why? Because it never has been done. It is not a part of their linguistic habit. Leístas, therefore, adhere to the Royal Academy's dictates when it comes to female direct objects, e.g., La vi el otro día or Las vi el otro día (I saw her the other day and I saw them [a group of women] the other day). As for what form to use to refer to indirect object(s), leístas and loístas are of one mind -- they must be expressed using le or les (which changes to se if followed by any of the direct object pronouns lo, la, los or las).