Why do some laptop charges have ferrite cores on them?
The DC power in your power cable is both produced by a switching power supply and consumed by a switching power supply, so while there is LC filtering at both ends, there is still more than just straight DC on the wire.

The ferrite core close to the laptop end of the cord is there to prevent electrical noise within the laptop from being radiated by the cord (which works as an excellent antenna).  This noise is generated by both the switching battery charge regulator/internal power supply and all of the other high frequency digital components in the laptop. While such radiation might not interfere with nearby data cables (though it might), it does tend to get regulatory bodies concerned (FCC, etc.)  From my experience, ferrite cores are generally used more to meet regulatory requirements than to maintain actual device functionality.