Why can't I sync using the USB cable?
At this time, only Apple's apps can automatically sync when connecting the iPad to a Mac with a USB cable.
iOS4 did introduce a new "file sharing" feature, which is mostly undocumented and tucked away under the App tab in the iTunes device window. However, in our opinion, this technique (in its current rendition) is not a friendly way to move data between apps on the Mac and apps on the iPhone. Having said that, we are keeping a close eye on the situation, to see how "file sharing" develops and we'll certainly consider implementation if/when we feel it is beneficial to our users.
One advantage of the current, wireless synchronization is that you don't need to plug in the iPhone in order to move family files to the iPhone or move edits/changes back to the Mac version of Reunion.