When to pull the plug?

At what point should comic publishers like DC and Marvel cancel a series that isn't as financially viable as was hoped? A few weeks ago, DC announced a few young comics were being cancelled early, including Green Lanter: Lost Army and Omega Men (which has been allowed to run for the original 12 issues), which was met with fairly universal backlash. Many people agree that cancelling issues after seeing sales figures for only a few issues and before even all of the issues that will be included in the trade paper back is unfair to the consumer and to a point the artist. But these publishers are companies with financial obligations, meaning that if a comic series is doing poorly, they should dump it and replace it with something that will be commercially successful.