Plasma can provide separate backgrounds and applets by means of separate containments per screen. It is up to the user to configure it this way though.
This is done to be able to offer a different wallpaper and/or widgets for every screen, which keep working even if more than one screen is visible at once.
so when viewing the desktop cube or desktop grid traditional desktop wallpapers don't really work that well: they think they are on desktop X even though desktops 0..N are actually being displayed simultaneously.
the "solution" right now in composition managers such as the one in compiz is to provide their own wallpaper painting and people turn off their "native" wallpaper compositor. this is nasty and not what we want in plasma: plasma wants to provide cool, animated, interactive backgrounds in ways that window manager devs just aren't very interested in.
a Containment has two primary purposes:
a) to contain a collection of plasmoids
b) to paint a background behind them
it is fully valid to want the same plasmoids on all your virtual desktops (using virtual desktops to organize your windows only) but different backgrounds. (b')
it's also valid to want the same background but different sets of widgets per desktop. (a')
it's also valid to want different backgrounds and different sets of widgets per desktop. (a'b')
an applet is only in one containment at a time. what we *can* do is have two virtual-desktops both show the same containment.