Why aren't those Moved Topics actually Removed from their original forums?

It never really occurred to as a big problem until recently. Spoiler threads that are not in the Spoiler Discussion Folder should not only be "moved", they should in fact be "Removed" from their original forum. So people who don't have the luxury of watching Inception that early will not be spoiled if they do not intend to.

Will there be any fixes to this?

I'm pretty sure there's an option when moving threads to leave a 'trail' behind. The purpose of this is that forum posters looking into particular threads may not know they've been moved elsewhere. A trail will give them a good idea where the new location of the thread is.

And, they'll probably be able to delete the threads just fine and move them without the traces. Maybe you can enumerate the specific threads they need to clean up?