What? Pokémon aren't items, they're living creatures... so what if they're "his" ? So if someone has a dog and he's abusing it, you'll say it's ok because it's his dog?...
Paul was obviously wrong and thanks to Ash he changed by the end of DP (as proved by him starting to thank his Pokémon after battles and his respect for Ash and Infernape)
Paul took the time to catch his pokemon and train them. Its established in the series that different trainers train their pokemon differently. What right did Ash have to decide that Paul's way was wrong?