From my 1st days in the Army we were taught to remove and clean the extractor and spring. Why now is S&W saying not to in the user manual?

Because you should replace the extractor pin every time you remove it.

I was always taught to place a couple of drops of CLP on the extractor and ejector, work both of them a few times with an empty case and wipe off the CLP along with the crud that worked out from around the extractor and ejector.

Page 28 of the M&P-15 manual says:

Press the rear of the extractor to check that the spring works (Figure 80). If there is no tension you must take the extractor apart and clean the parts.

Page 28 continues with instructions on how to disassemble the extractor from the bolt if the extractor doesn't have any tension.

S&W Customer Service might know the reason for having owners inspect the extractor rather than just routinely disassembling it. My guess would be that they have determined the likelihood of having the extractor fail in the environment most civilians use their rifles is pretty small while in the hands of untrained personnel, there is a greater likelihood of damaging or losing a part through repeated disassembly.

I seemed to have skipped that on page 28 thinking, I knew better. Since the armorer always inspected the weapons, it was the 1SG telling you can go home.

"Replace", as in put it back where it came from, or put in a new one?

Yeah, I was wondering about that comment myself . His statement only makes 'sense' if he meant "install a new pin everytime", but I have NEVER heard that before.

Exactamente!

It's a pin that's shorter than it's bore, both
ends are tapered, and that part of the bolt is contained within
the carrier--so it's got nowhere to go, even if
it wasn't retained in position by extractor
spring tension.

I couldn't guess how many times we pressed out
the pin and cleaned extractors on A1s...and we didn't
even have rubber inserts, or O-rings, and don't recall
seeing or hearing of an extraction failure, ever.