How activate for all

i have a answer from a akismet developper
There are a few ways of doing that.

You can create a quick plugin that checks if the akismet_comment_form_privacy_notice option is set, and if it is not, set it for the current blog (note: update also works if the option was never set): update_option( ‘akismet_comment_form_privacy_notice’, $state ); where $state is either display or hide.

Or you could write a script that loops on your blog list, and set the same option, in one run.