I have tried that 10 times it doesn't delete anything in the .mobilebackups.TRASH.
It's annoying I have just lost 14 gb of storage.
I have been tinkering around on my system a bit and I think if you just wait a few minutes you will be okay.
Here is what is going on (based on my experiment).
MobileBackups are stored at /Volumes/MobileBackups and when you turn off Time Machine either through the GUI or the command line it immediately renames /Volumes/MobileBackups to /Volumes/MobileBackups.trash and shows the trash in the Other category and not Backups. If you wait a few minutes the /Volumes/MobileBackups.trash gets deleted and Other reflects this.
I had about 3GB in MobileBackups and it took around two minutes to delete it. Try checking again in a few minutes and see if it is gone.
If not maybe try turning Time Machine on and off again then try a reboot and wait a bit.