It's all about the Ad revenue. Something easy to overlook about the 'new' Flickr. If you look at the Help page, you'll notice that the Ad Free account, for $50 per year, includes:

• No ads in your browsing experience

However, the Pro account included:

• Ad-free browsing and sharing

By having a Pro account, people who I directed to my pages were not subjected to ads. That's not going to be the case with the double-the-cost Ad Free account - for your $50/year, you won't see ads, but anyone who follows a link to one of your images will.

I think the $50 price is intentionally too high - they don't intend to make money from Ad Free, they intend to make money from Ad Sales, so they've priced the opt out option high enough to limit subscribers, and thus boost ad revenue.

It will be interesting to see how ad placement evolves, too. Given their plans to 'not screw up' Tumblr, but also to 'monetize' it (those are perilously close to mutually exclusive, IMO), I expect ads to migrate from a sidebar to inline placement (between your images) in the future.