Unfortunately, you can't use "in:", "is:", "label:", or "has:star" in filters. Filters are applied before these things are determined. If you try to, you get the following message from GMail:

Filter searches containing "label:", "in:", "is:", or stars criteria (i.e. "has:yellow-star") are not recommended as they will never match incoming mail. Do you still wish to continue to the next step?

It's worked for me for years: http://i.imgur.com/bUB3u.png

This works because this filter will match every mail, not just the mails that have the spam label. In the end it does what you wanted but the error is still right.

Odd. I've never gotten it to work. I tried again today just to see if they may have changed that, but still gives me the error.

I think I remember seeing the message actually. You just blow past it and setup the filter, and it'll work, I think.

Is that the same as marking things "Not Spam"? As in, does it lower the quality of Gmail's spam filter?