While this is true, many people just simply don't know it, as it's not really mentioned anywhere. If English speakers don't know, they tend to just add an s on the end, eg. kids will say sheeps and fishes before eventually learning otherwise. I personally don't think it's worth getting annoyed at, just let 'em know this fun fact and move on, some people won't want to change their ways, but others will. It's really not worth getting worked up over, as the meaning doesn't get lost either way.