Perhaps so.. but I think if you look at it from the perspective of the average user; photographers are more likely to care if someone downloads their photo (whereas someone who just snapped a shot and posted it is more likely to be flattered as opposed to expecting it to provide an income), and more likely to change a default setting (or even to think of checking to see if there is a setting).
So I can see where from Google's perspective it makes sense.
I do think that Google might do well to take a few common settings, particularly regarding sharing, and throw them in as questions available to answer when first coming in to Google+.
Given that by default, photos uploaded are not visible to anyone, and that you have to choose to share them, from that perspective it is already opt-in to a degree. The changing of the setting is just letting you 'opt-in-but-not-all-the-way-in' as it were.