I noticed that the standard WP 3.0+ widgets don't have any problems when dragged-&-dropped into its default theme's (TwentyTen) sidebars (via Admin > Appearance > Widgets), especially when they are made available for more than one sidebar. However, this isn't the case for WP e-Commerce plugin's widgets. Once, say, the "Shopping Cart" widget is dragged-&-dropped into the "Primary Widget Area" of TwentyTen's theme, then this widget is no longer made available for other sections.
What could be causing this to happen? Please advise.