Here is a hypothetical:
1. I post a photo of a dinner table at a restaurant that shows some fizzy drinks in glasses.
2. SodaStream wants to post an ad on Instagram, and my photo gets selected kinda like a stock-photo would be selected because it is drink/food related, and it gets shown somewhere on Instagram's website.
Result: my life is not negatively impacted by this in any way, and the world continues turning.
My reaction: If ever saw this happening, my reaction would be "Whoa cool, that's a picture I took! And now tons of people are seeing it, instead of just the 5 or 6 people that normally see my Instagram photos. I'm internet famous, haha!"
The blogo-sphere's reaction: "RAAAAAAAGGGGGGEEE!!!!!!!!!"
I ask: "Seriously, what is the big deal here?"
They respond: "Just shut up, and RAAAAAAAAGGGGEEEEE!!!!"
The whole point is not about the majority of dinnertable photos - for those content producers who make quality photos that everybody (not just your friends) want to look at, the content rights are an important issue. And these few users do represent a lot of value, and their leaving would hurt both Instagram and the content consumers.
For some people, its not cool. And, being unable to buy a controlling stock option in the company to make it obey their wishes, they used the lever of public dissent.
Looks like it worked, too.