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.