Calls for Papers / Articles is the classic reason. The IEEE and the ACM might have proper mailing lists for that sort of business ... most academic fields do not.
Edit: I think they could lower the rate more and push mailing lists, but on the other hand a lot of users simply wouldn't notice that they're rate limited. Which could lead to entirely different brand of lulz.
Calls for Papers / Articles is the classic reason. The IEEE and the ACM might have proper mailing lists for that sort of business ... most academic fields do not.
Edit: I think they could lower the rate more and push mailing lists, but on the other hand a lot of users simply wouldn't notice that they're rate limited. Which could lead to entirely different brand of lulz.