as described this feature does not work. There is not even an error message that this doesn't work. In my opinion every tags should have a default folder of some kind where files can be dropped.
I don't see how this is supposed to work. Tags are just properties on a file, all we could do is add a tag to a file when you drop it into tags:/// but that is a bit cumbersome, isn't it.
actually this already used to work as described here: * https://userbase.kde.org/Nepomuk/kioslaves/tags drag & drop on a tag:/ - folder would give the dropped file the associated tag. Now it doesn't work anymore so I would regard it as a regression.
The tag ioslave is provided by baloo these days, moving there.
Actually there was already a report for this. *** This bug has been marked as a duplicate of bug 340098 ***