I’ve been recently thinking about it, too. I think that since me replying to you via webmention requires my site to actively ping yours, it is safe to presume that I have configured my site and composed this reply in this particular way knowingly and with intention, so that my reply will be given to you to publish on your site. It is like editing a wiki or putting a post on a forum: a person doing it is supposedly aware that what they’ve written will be published on this wiki or forum or something. With webmentions the same logic can be applied: yes, the request itself is hidden in site-to-site protocol, but it’s still a deliberate act at its roots. Backlogging replies from silos, however (like Twitter via Brid.gy), is a whole different story, and I’m not so sure the same logic can be applied there.
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