It is hidden under the little ⓘ in the bottom right corner. That is a very meagre interpretation of the attribution guidelines. To help combat the idea that a map must be made by some megacorp to be viable, visibility of OpenStreetMap is important.

ellenhp: Why not start with the attribution toggled open, or some alternative way to highlight the map data's provenance?

edit: Attribution is now shown toggled open at page load, resolving this issue.