Why 9 speed front derailleur cannot handle 10 speed rear derailleur?
The front derailleur doesn't care what the rear derailleur is and vice versa. The front derailleur is matched with the front chainring sizes, number of chainrings, mounting type of the front derailleur (a property of the frame) and the front shifter.

The rear derailleur is matched to the shifter (for indexing; the shifter is matched to the cassette), and the capacity required (which is dependent on the amount of chain slack it needs to absorb). 

The point of saying that is so that you can get someone to buy a new front derailleur unnecessarily.