http://www.macrumors.com/2013/01/09/apple-now-locking-screen... gives an explanation for the 'why':
"This small but important update shuts down a widely used scam tactic, where
developers would upload game screenshots to get an app approved by Apple and
then switch them out with screenshots from another popular app.
The scam tricked people into buying fake apps with screenshots ripped from
another, more popular game, or a game that has not been ported to iOS at all."