Sorry, it's a personal irk. Cancer is a horrible, concrete, real thing that I have experienced far too much of in the past year, and to see it compared to something relatively trivial and abstract makes me immediately dismiss the author's argument.

Edit: Upon further reflection, I guess my point is that "x is cancer" is far too powerful of a metaphor to just throw around. In my opinion it should be reserved for things that have all of the personal heart wrenching connotations that the word implies for people who have experienced it.