Sorry Robert I seem to be contradicting you a lot lately
Honda's specs for inspection and replacement are driven by a few factors, including what the engineers expect the plug life to be, which can affect engine performance, as well as how a worn-out or incorrectly gapped plug can affect exhaust emissions. As far as the Environmental Protection Agency is concerned, the spark plug is indeed part of "the emissions system" and they are very concerned about ensuring customers recognize this fact and keep the engine's emissions output within the certified limits.
While the EPA isn't going to start inspecting and writing "tickets" for homeowner's mowers with incorrectly gapped plugs
eek
, there is a general push within the small gas engine industry to improve and extend emissions performance, and part of that is making sure existing engines comply to EPA-certified figures for as long as possible.
Straying a bit off-topic here, Honda did announce its first fuel-injected portable generator a few weeks ago; I wonder how long it will be before we see fuel-injection on <200 cc vertical shaft mower engines?