Because there isn't an Air KWin decoration yet. The description may be a little confusing but "Plasma integration in KWin" means that KWin now uses the Plasma theme (Which by default is Air) for displaying popups such as alt+tab window switching, desktop effect text, etc. that all used to be unthemed.
There is currently a decoration under development that allows for SVG-based decorations that will be included by default in KDE 4.4 called Aurorae which should make it very easy to create an Air decoration. Aurorae is currently also available for KDE 4.3 via KDE-Look.org .