Most people who buy them have relatively small yards, if you mow 3-4 months out of the year and it takes you 1/2hr to 1hr a week you put on at most 16 running hours a year (I mowed my 75x150 large town lot in 40 minutes on a slow day). If they figure those engines will go 1-200hrs between actually needing an oil change that equates to 6-10yrs of mowing. I have to agree with them that running those engines low on oil is going to kill them much faster not changing the oil. A top off will replenish the additives and the heavy gunk just settles to the bottom of the case, most of the lubrication is in the form of mist in those engines anyway.
I've picked up plenty of old mowers that probably haven't ever had their oil changed and they run just fine once the carbs cleaned out, some of the oil I've dumped looked more like asphalt coming out of the drain plug. Oil so thick and black that it comes out in glops.
Personally I prefer to change my push mower oil every other season but I can see the appeal of advertising things this way.