So how come vegetarians don't eat ice cream?

"Simple" questions like this usually turn into extensive explanations, not because vegetarianism is complicated per se but because broader issues need to be taken into account. The following three are frequently involved:

Speciecism

Human nutrition

Planetary conservation

Some vegetarians don’t eat ice cream because it is made from milk, which is exactly the same when it comes to fellow earthlings exploitation. In some countries this exploitation is specially disturbing, as milking cows endure extremely deplorable conditions.

Milk is loaded with potent nutrients meant to help mammals grow strong and healthy. No mammal though keeps on feeding on milk past a certain age, and they certainly don’t drink the milk of other mammals. Humans are the only species that do this, which from an educated nutritional point of view is no longer necessary.
http://www.vegansociety.com/html/food/nutrition/calcium.php

Oh yeah, we’re depleting not only our food supplies but everybody else’s in the planet too. As the invasive species that we are we need to modify our eating habits pronto, if we want to continue having a decent planet to live in to begin with.
http://en.wikipedia.org/wiki/Invasive_species


Aren't humans omnivores though?

Humans are indeed omnivores. Such fact though doesn’t mean that we still have to eat meat in order to be healthy, but only that meat has been part of our "menu" practically since the dawn of our species. Fatty acids in meat in fact, according to some serious studies, contributed to the development of human intelligence, we would’ve turned into something entirely different without these nutrients.
http://www.fi.edu/learn/brain/fats.html

Nowdays however is different. We can manipulate our nutritional needs to the point where animal products are no longer necessary. Ethically, we’ve evolved to the point where we can contemplate and assume responsability for the consequences that our eating habits have brought upon the planet and the rest of the animals we share it with. It’s hard for many to give up that ham sandwich, I know, old habits die hard. Unfortunately even harder consequences will have to be dealt with now and in the future if we continue putting off thinking outside the box...