Thank you @gerhard, it is very useful to know that this is actually a feature. It makes sense when you know the reasoning behind.
The idea is to embed posts from several external sites. I don’t have direct access to the administration of these sites, and even the possibilities of the owners of those sites might be limited. In any case, the JavaScript plus the possibility to configure a CSS selector does provide a flexible path for multiple situations.
For what is worth, having the possibility to define a RSS feeds per origin would simplify this problem too, since the RSS filter also acts as a selector of sorts. However, I am realizing that this use case might be too niche nowadays – see Embedding: one RSS feed for each host.