I think it's just a marketing thing. Those "iPod compatible" blurbs are just advertising to the non-audiophile consumer and have been around since before iPhones were introduced.
A person who isn't really knowledgeable about audio equipment would go "Oh, I have an iPod, that means I can use. I'll buy that one!"
Of course, if it says iPhone compatible, it generally does mean it has a recessed jack.
But in general, it's just a way to make people aware that the most popular audio device out there is compatible. Kinda like saying MP3 player vs iPod, even though an iPod is an MP3 player, some people find it essential to distinguish the two.
I also generally don't see these iPod compatible signs on any higher end products, usually < $30 type products. I think UE may be one of the exceptions.