Dropping files into search results playlist, or any dynamic playlist, is a feature that should be disabled by default.

The reason is that it's not clear to especially novice users that it's a dynamic playlist (like a temp folder or trash/recycle bin that may get emptied), not a normal playlist. And because it's the playlist that's often selected after a search, a user can use it for a long while, dragging and dropping from an external file system manager or search window, believing it's just another playlist, expecting it to persist. When the user makes a new search, the contents are lost which can be the result of a lot of work. The user will wonder what went wrong?

The design logic went wrong.

This has happened many times, even to seasoned users and always it's been what was not intended. It was just something you didn't pay attention to even if you knew it. It's one of the main goals of software design to prevent simple human errors, like not allowing you to lose data inadvertently (while allowing it if that's your intention).

To prevent this, an option must be checked to allow its manual use. If it's not checked, dropping a file on the search results playlist or any dynamic playlist

a) does nothing
b) inserts it to the next or last active playlist (and perhaps notifies about it)
c) tells the user "Are you sure? This is a dynamic playlist which loses anything you added to it when it's updated, that you can drop files and folders on only after activating the feature. Would you like to activate it now?"

That makes its use intentional. Currently, because there's no option or even a warning, unintentional loss of data is possible. It's like a text editor that allows typing to the search results area which looks exactly like the normal text area, but it will be erased when you search again. It may fool you just once, but even that may be enough to lose an important chunk of data. If you for some reason need to do that, you need to know what you're doing, and activate the feature. Dangerous, potentially data-losing features should have a safety that you must disable before use.