The reason for disabling filters on sources is that a source might be used in multiple different topics which want to apply their own filters to all unfiltered articles
The following describe the steps creating a topic that should include a specific source.
Create a topic using the "+Topic" function either in the admin tab or from "Topics I manage".
Select the "manually select sources" function
Enter the name for your topic and select the "Create topic" function
You will be transferred to the topic administration view. In Existing sources for this topic, select the “+Add new sources” function
In the select sources view, you can find the feed by title or going directly to the catalog and category where you added the source into.
Once you found the source select the “Add” function to the far right of the source line. When you are done select the “Save and Add” button at the top right (it should show now “Save and Add (1)”.
You will see now the selected source displayed in the admin view
Select the “Filters” tab to enter your filter.
Now you can enter any of the Boolean filter terms. When you are done select the “Apply” button and you will see the filtered results on the right.