interfacing MMA7361L with PIC ADC
Page 3 of the datasheet:


  The device can measure both + and – acceleration. With no input acceleration the output is at midsupply. For positive acceleration the output
  will increase above VDD/2. For negative acceleration, the output will decrease below VDD/2.  




It has to provide the offset, because acceleration can be positive or negative, and a 1 g negative acceleration would need a negative supply to get the -800 mV output. It's a lot cheaper to offset the output so that it's always positive, and subtract the offset again in software.

Further reading
Implementing Auto-Zero Calibration Technique for Accelerometers, Freescale application note