That's how it is suppose to work. THe MAF also often has a temperature sensor in the assembly, and the engine computer get the air temperature and mass flow data from the MAF assembly. These two data points allows the computer to determine how much fuel to inject. The Pre-cat O2 sensors then measure the O2 content of the exhaust gases and, if it is too high, indicates a "too lean" condition. If it is only a little bit "too lean," the computer will use the O2 data to adjust the fuel amount, adjusting a parameter called "fuel trim." But if the lean condition it too great to correct with a "fuel trim" adjustment, it will set the "Too Lean" OBD-II code and turn on the SES light.

P.S. The post-cat O2 sensors purpose is simply to make sure the cat is working to ensure emissions compliance. The computer compares the O2 readings for the pre-cat and post-cat O2 sensors and uses this data to determine if the cat is really doing anything, i.e., defective post-cat O2 sensors will not impact engine operation or performance.

Hope that helps.