Maybe I failed to ask my question correctly. What I am asking is why the need of a different charger since the bike itself does not have anything in the charging system related to or different to accommodate the lithium battery? I am looking for an electrical explanation.
The bottom line answer to your question is that different batteries with different chemistries require different charging modes.
I'm not sure I can give you the technical explanation you want. Automotive charging systems simply put a higher voltage across the batter terminals and the battery's chemistry stores the energy. This constant voltage (well, it will vary somewhat due to loads and engine speed) is relatively benign - it will not break down lead sulphate on the plates, and it does not do a complete job of charging the battery.
So called smart chargers for lead acid batteries not only provide the constant charging voltage, but then decrease the voltage to top off the battery (there is also an equalizing stage in which, I believe, the charger drops the voltage even more to allow the battery to stabilize). Old fashioned trickle chargers, which simply charged at a lower current would eventually overcharge a lead acid battery and destroy it - or at least cause electrolyte loss by dissociation. This is essentially what the car or bike's charging system would do if you drove it 24/7 - though you might not have that problem with modern maintenance free batteries.
Somewhere along the line, these smart chargers for lead acid batteries gained a desulphating stage in which the charger sends pulses of higher voltage (than the normal c. 14 v charging mode) to the battery. This breaks down the lead sulphate that might have formed on the battery's plates. This pulsed voltage will kill a lithium battery - or make it very unhappy.
In normal every day use most guys don't use a LIFe battery tender - it shouldn't be needed. And as stated above by other posters, these special chargers have circuitry to equalize the cells, prevent over charging, and maintain the battery in peak condition. This is not done by your bike's charging system.
I suspect the real answer to your question will be found elsewhere on the web in battery manufacturer's websites and technical discussions about battery chemistry and their charging requirements.