The problem is made worse when new users ask their first question in those tags and get a comment saying "could you put your code in a fiddle?" I imagine next time they go to ask a question, they just create the fiddle and link to it, thinking that they're being a good SO community member and following advice that the commenter gave them the first time around.
I know this has been discussed before and been deemed a problem, but I haven't really seen a meta answer that proposes a solution. I'd like this question to be focused on figuring out ways to discourage "link only" questions that would be valid if the code was in the question itself.
A few thoughts to get things going:
The only thing in the FAQ that I can find that remotely relates to this is the "What questions can I ask here" section:
We feel the best Stack Overflow questions have a bit of source code
I propose adding a more explicit guideline about just linking to external sites, something like:
Feel free to link to a sandbox site so that potential answerers can easily interact and modify your code, but don't forget to include your source code in your question as well!
It seems like it should be easy enough to warn the asker that they have no code in the question. I know that there are probably some questions that are valid without including source code, but a warning before posting wouldn't hurt:
We noticed you didn't include any source code. Remember that just providing a link to the code in question isn't enough. Please provide the code you're having trouble with inside the question text.