One reason for the grocer's apostrophe and so forth can be that someone in a hurry has made a mistake. The other one, which I suspect is very widespread, is that the cunning grocer puts in the mistakes deliberately, knowing that every now and again a couple of pedants will be walking past, notice the error, notice the notice, and a purchase will be prompted.

As for possessive apostrophes: "They add absolutely nothing to sense or clarity" but "If you get marginal cases where the meaning could be ambiguous..." (Paul Burke). In other words that first bit is not actually true.

George Bernard Shaw huffed and puffed about this, and insisted on the offending apostrophes being removed from stuff he wrote.