You aren't missing anything. When you stream a song, it's downloaded to cache memory in the background. iTunes Match is designed to automatically remove the oldest and least played songs from cache memory as your device begins to run out of space, but isn't something you can do manually. The only way to delete these songs is to restore your device.
As you've noticed, songs that you downloaded to your phone (as opposed to streamed) can be manually deleted by swiping on the title and tapping delete.