I kind of feel like the progression is right. It's better to learn how to use most/all of your other tools first, then your aggro crutch so that you're not over reliant on it. Also, during the leveling process a lot of people (especially new players) are doing the MSQ which involves a fair amount of soloing. Best to give them the more offensive stance first so they can solo a bit easier/do a bit more damage, then give them the Shield Oath for when they start getting into harder dungeon progression.

Outside of that? Even if it doesn't work out that way, another reasoning might be to try to get DPS to learn to use their own de-aggro tools and control their hate output. Yes, one school of thought is that DPS needs to be able to go balls to the wall to get content done faster, but another school of thought is that it's an irresponsible DPS that can't control themselves and rips aggro away thoughtlessly without using the tools they're given to mitigate it.