Well, if you are the editor and you are paying for the production and publishing of the book, you have to decide if your personal participation as a contributor would enhance or detract from the overall work.
In some markets, if editors participate as contributors, it will be seen as self-serving. I would guess that more literary markets might lean that way, but I don't have any hard data to back that up. And it would depend on whether the editor is also the producer and publisher. If the publisher hires an outside editor, the inclusion of the editor's contribution may be part of the compensation and perks offered for the editor's time and talent to edit the anthology.
In markets where nonfiction anthologies are done for business purposes, I have often seen the editors contribute a chapter. In this instance, the editor may also be the producer/publisher, and they want to get some marketing value out of their investment.
So there's no standard policy, and it really depends on the market and the project itself.