I bet that such a function could be implemented.
But then again: you would have to get into the habit to press the CTRL-key.
Wouldn't be just as easy to get into the habit to select the files first? The good thing about that way would be that no extra effort would have to be spent on programming. And you could start training straight away as the function is already there.
And it is one of the basic functions of object oriented user interfaces: select the object first, then manipulate it.
But I see the point - perhaps disabling the tag panel when there is no file selected would be a nice feature. But this idea has been around for some time, see e.g. here: