So...the difference is that the SSEi'* don't have it...and the SE/SLE'* do. And this is the california emissions thingy? It could be
1) performance related...perhaps the non-aerated were found to perform better with these, thus they gave them all one in these years

or

2) demand related....perhaps there was a larger demand for SSEi'* - in which case they could special order cars with or without these. Perhaps the demand for the SE/SLE'* were minimal to the point where it was easier to manufacture them all with.

im not sure, there is a sensor and a vacuum diaphrom/modulator that go along with it that are mounted on the rear of the egnige, just peaking above the engine cover, so i think i would set a code if it was removed, however, it will also leave a big hole in my crossover pipe for the exhaust.

If noone knows what the purpose of an air injection pump: The pump pushes fresh air into the exhaust to help the catilyst in the catalytic converter work more effectively at removing harmful pollutants.

Yes, but it only does that on start up. You could probably have the codes deleted from the pcm and then remove the air pump from the vehicle.