Why doesn't SIGKILL terminate a stopped program (yes)?

I'm using Ubuntu 14.04 and I'm experiencing this behavior I can't seem to understand:

1. Run the yes command (in the default shell: Bash)
2. Type CtrlZ to stop yes
3. Run jobs. Output:
   [1]+ Stopped yes
4. Run kill -9 %1 to stop yes. Output:
   [1]+ Stopped yes
5. Run jobs. Output:
   [1]+ Stopped yes

This is on Ubuntu 3.16.0-30-generic running in a parallels virtual machine.

Why didn't my kill -9 command terminate the yes command? I thought SIGKILL can't be caught or ignored? And how can I terminate the yes command?