I had English English teachers at the International School of Geneva I attended from grade ten through twelve. Most of the students were American. This rule has an ocean between it. The English do not use s's. And, I believe that some American 'rule books' make an odd exception for Jesus. It is never Jesus's sandals. Must be a remnant of the Biblical translation wars.
Funny - you know, where I see the most abuse is words ending in a vowel. Every other plural might have been done correctly, except when ending in a vowel. For example, "the drunks and wino's were out that night" or "10 percent off all subs and pizza's."
The apostrophe on a plural word drives me up the wall. I've been known to go on tirades about it--mainly because I see people with high-paying jobs doing it. I could have done a better job writing memos when I was in first grade, apparently.
I, too, cannot but marvel at the thoughtless incongruity wherewith they who have been bestowed with Albion’s fine tongue do seldom refrain from trampling upon its venerable orthography, and in no slight measure does it grieve me to behold yon philistines gralloching the umbles of the defenseless heifer that is Dr. Johnson’s fair progeny; such is this, my woe-stricken crestfallenness when I gaze upon the embers of the whilom towering temples of your beauteous idiom. :(