I thought that too at first. But the NYT op-ed does a better job of explaining.[1]

Basically, they think that the NSA has the ability to index anonymous readers with pages visited, and anonymous editors with pages changed. Then if the NSA is sending that data to a bunch of governments around the word (Egypt, Israel, the Five Eyes, whoever else), dissidents around the world are at risk of being caught for browsing/editing Wikipedia for the crime of being opposed to the government in power.

[1] http://www.nytimes.com/2015/03/10/opinion/stop-spying-on-wik...