Also, most consumers are used to using cards those cards preformatted with the 'archaeological' fat system (and its many headed variants). These don't support decent OS-native security permissions (well, short of custom-encrypting everything written to them).
That opens up an easy pathway for one app to get at another apps files.
In short it brings us many of the potential doomsday scenarios MS-Windows also has. Yay!
OTOH, how hard can it be to demand a proper format like ext3 (for example). Therefore I'm sure marketing does come into it as the above posts state. However, the security issue is real.