Tag warnings (or "tag tips") are yellow warning boxes that pop up whenever you go to ask a question with a certain tag (e.g. [regex] or [sql] on Stack Overflow). See the feature request that caused this to be implemented here.
The problem is, tag warnings show up if you're editing (sometimes, such as when clicking on "edit tags"):
It makes no sense to trigger tag warnings during an edit, especially when it's someone else's post. Tag warnings are specifically designed to give advice to the asker, so there's no advice there that an editor who isn't also the OP would be able to act on (usually).
Alternatively, it makes no sense to trigger them so inconsistently when editing. If you did have advice for editors in there, most would never see it.
Here are some questions with tag tips: