First of all, I don't know why you're so obsessed with the batting average, given that Walker is hitting for no power, while Vladdy in his first month was at least hitting for some (.181 ISO in his first). Walker also has an awful K/BB, Vlad had a 20% K rate and 10% walk rate, both indicative of a much better approach. And then the easier to justify part, the Blue Jays were not that good of a team and could afford to give their top prospect more rope, the Cards want to be competitive, and can't have the worst defensive outfielder in the majors who's just average offensively to drag down their production when they need wins.
This argument makes zero sense. Vlad dominating the minors means there's nothing else for him to prove by sending him down, it's just a waste of development time for him. Walker could actually use some more development, as his hitting approach clearly has some flaws and the defense needs major work. He did not dominate AA, he merely did well at it at a good age. Julio Rodriguez and Kelenic both had far more MiLB success at younger ages, and the latter got sent down multiple times so I don't get the point that he was babied, at least Rodriguez and Kelenic weren't butchers in the outfield.
It's pretty clear Walker is not a finished product, he needs more development both defensively and offensively. The Cards thought he was more of a finished product and had the athleticism to learn RF on the fly in the MLB, obviously this has not been the case.