Well the thing to remember that a lot of things in Pokémon were inspired by yokai, as for the question from various interviews at its core many Pokémon were designed in such a way as to be your friends.
Now sure the Poké balls could be used to tame Pokémon in such a way as to prevent say an Arbok or Serperior from eating its own trainer, there have been cases of pet snakes trying to eat its owner for example this one woman thought she was bonding with her pet python when in actuality it was sizing her up to eat her. While one could argue that Poké balls enslave Pokémon when you look at some of the concepts you began to get a better understanding. In many regards Pokémon are one of the many modern takes on Yokai.
What you need to under thans is that it it can be argued that the onmyoji (who were basically Japanese wizards) who are connected to Onmyōdō (the study of Natural science and magic or simply put Alchemy) now as for Pokéballs one could argue that they are parley insotred by the Ofuda (paper slips) that some sores say that that the onmyoji used to capture and sumon varius Yokai.
@ Rayancaleb While you may have brought up a fair point about the moral aspects of Pokémon training however I think the following vidos should give you some idea on some of the concepts of being a Pokémon trainer. as for the question of
moral aspects of keeping Pokemon inside Pokéballs one theory is that the inside of the pokéball creates the perfect environment for them, one could theoretically suggest that the inside of the Pokéballs are a house in an interconnected world similar to the Pokémon world shown in the Mystery Dungun world a plaice I once called the training grounds and briefly showed in anther story when a psychopathic Aerodactyl named Drago (which is based off my Aerodactyl in the ASB which I organelle named Drago I since changed his name to Lord Amber.) attacks Charles (actually the charter of Ryu) Please do keep in mind that this is my own opinion on the matter.
The History of Yokai in Japan! - Gaijin Goombah
Onmyoji, Japan's First Monster Trainers! - Gaijin Goombah
What Happens Inside a Pokeball?
What Happens Inside a Pokeball? Part 2
Is Pokémon Ethical? - Video Game Theory - Tamashii Hiroka