Why do people who have had botox, have such phony foreheads?

Poor Technique. Botox must be injected correctly to produce a natural appearing forehead. If the injector does not place it into the specific muscles causing the forehead wrinkles, an unnatural appearance will result. Not uncommonly, too much Botox is injected, causing a flat and immobile forehead.
Overdone. If injected correctly in the right doses, you should have a very natural appearance with some movement.
Bad injection. Like a zen parable, the best plastic surgery (from Botox to facelifts) gives results which look completely natural...So you would never know someone had anything done. When properly done, Botox should smooth wrinkles, but still leave a natural appearance with enough movement to allow normal expression. Phony foreheads can be avoided with good technique.
Not just Botox. Most likely it is alot more than just Botox alone that is responsible for this appearance.
Botox. What do you mean by "phony". Botox done well by an experienced physician is only noticed as an individual who is aging well and retaining a youthful skin texture. But there are many other factors that are involved including not smoking and strict sun protection just to name a couple.
Selective Botox. Selective injection of Botox into the upper facial muscles of the forehead will preserve some motion while calming the overactive muscles. With artistic use of Botox the deep lines and dynamic muscles that cause wrinkles can be smoothed while leaving the ability to show emotion. This is good botox. www.artofplasticsurgery.com.
? Not sure what you mean by phony. If you are talking about the forehead not wrinkling that's kind of the point.