User blog comment:Callofduty4/Why is it a sin for Treyarch to rip some aspects off of Infinity Ward but OK for Infinity Ward to rip off some stuff from Treyarch?/@comment-1721748-20091224074118

Infinity Ward does not make a new game everyday.

Treyarch does not copy Infinity Ward. They're the same series, they have to use the same engine (not have to, but it'd be weird), the same control setup, and all that stuff. WaW was more recent than Modern Warfare.