I don't care what their reasoning is, I know it's not out of the goodness of their hearts. I just like that web developers have to be mindful about making sure their stuff works on a browser that isn't Chromium. Myself and Apple happen to want the same thing but for different reasons. As of now, the effect is still net good for the open web.
I assume they'll be forced to allow other browser engines on iOS at some point in the next few years, at which point the floodgates will be opened for Google to have free rein on web standards and further cement themselves as owners of the internet.