Power = Voltage x Current (P = IV)
Voltage = Current x Resistance. (V = IR)
Since the cable has a resistance that is non-zero and a current flowing through it, there is a voltage drop across it, according to the second equation. Because of this voltage drop, the voltage received by the mouse is slightly less than the voltage supplied by the USB port.
Increasing the length of a cable increases its resistance, and correspondingly, the voltage drop experienced. Logitech might be concerned that if you use a long enough cable that the voltage drop will be sufficient to keep the mouse from charging.
Charging a battery depends on a difference in voltage between the power supply and the battery. For example if the battery provides 3.7V at full capacity, you need a power supply with >3.7V, such as a 5V USB port to charge the battery.
All that said, you can usually get away with 6 feet or so of USB extension. It might slow down charging very slightly, but I regularly charge my phone and iPod from an unpowered USB hub which is connected to my machine with a 6 foot extension.
Check what voltage the mouse battery outputs. The closer it is to 5V, the more detrimental the effect of lengthening the cable.