The "Why" of this can probably only be answered by Microsoft.
Here's my take on interpretation:
When Flow started out, the HTTP request and HTTP trigger actions were not premium. People discovered that they could use these HTTP requests to do things that otherwise required premium connectors to other services. So, after only a short while, the HTTP actions became premium, too.
The exception is the action that sends and HTTP request to SharePoint, since SharePoint itself is not a premium functionality.
Seeing that SharePoint is deeply integrated into the Office 365 universe, and seeing that it has a very close link to Teams, it doesn't surprise me that aspects of Teams can be addressed with an HTTP request to SharePoint. They're in the same family of O365 apps.
However, using this HTTP request to SharePoint action to send commands to services outside of the Teams/SharePoint realm is a different licensing story, and that's probably why you can't use the free HTTP connector to do that.
It's a bit like the Office 365 family with a bunch of kids in a restaurant. When sister SharePoint says to brother Teams: "Pass the salt, please.", then that doesn't incur any extra cost. But if you order a meal for they guy at the next table, then you'd have to pay for that.
Now, don't spin that metaphor too far, it doesn't correctly reflect the licensing model. And also don't ask why it is possible to do X but it isn't possible to do Y. There are probably a lot of aspects that play a part in this scenario, but licensing comes to mind first when talking about HTTP request.