Can anyone explain?
Quite simply, they're different batteries and not cross-compatible with eachother. The "blue" C-S2 battery won't work in a device that requires a "green" C-X2 battery, and vice versa.

Is it blackberry way of making more $$ and forcing us to upgrade?
Forcing you to upgrade? Nonsense. Your 8800 didn't come with a battery of its own?