I guess that misunderstanding of what a milkshake is comes from language differences. When I was a kid, my mum sometimes made me a Bananenmilch ("Banane" and "Milch" are, as expected, "Banana" and "Milk") so I guess what you guys know as a smoothie. When we got milkshakes here (remember, Austria, where we speak mainly German, but we imported quite a few English words - which leads to weird people whining about how our language is dying, but that's not for a positive thread like this), called "Milkshake", I assumed it was just a fancy name for a "Bananenmilch" or "Schokomilch" (chocolate milk). Until now, I always assumed using ice cream was some sort of lazy way, instead of using a real fruit.
I also didn't know that smoothies can include milk. I always thought they used water. Never really liked them much, the ones I tried where too thick, and it always felt like there were still small fruit pieces in there (maybe our blender was just good, but my banana milk was always without fruit pieces, the way I like it). Or, rather, I don't buy them, but simply make them myself. No need to add any sugar to that stuff, fruit and milk is sweet enough as it is.