On one hand, excluding all results isn't as useful to the user as including all results.
On the other hand, I think including all results may make the filter less intuitive.
Another consideration is multiple selection filters. Often, a way to "select all" is convenient to users.
It's important to understand what the functionality of a Filter is.
The reason why a user would apply a filter is to weed out results that they don't want.
If no filter is applied, the user expects to see all the results.
As far as the "showing all results seems non-intuitive" is concerned; that is a whole different use case on its own. That depends on how the results get shown. Is it a list that gets populated as soon as you open a certain page? Or does it get populated by a user when they use a Search functionality? But I don't want to get into it because it's not in the scope of this question.
