When I first traveled east on business to New York and the east coast many years ago, I found it highly unusual to find sliced pieces of sausage on thin crust pizzas there, rather than the sausage chunks and hunks that I was use to back in my native Chicago. What I've learned and heard about since then is that slices of sausage on pizzas exist primarily in the eastern part of the country and other places that follow a mostly New York/NJ style of pizza tradition. In the midwest -- and actually most of the country, I think -- sausage is put on pizza in hunks, clumps, or balls, as annpeeples mentioned. And in some places, crumbled sausage is also found (i.e. spread out like breakfast sausage is how I interpret crumbled). The reason for having slices of sausage, I was told, was twofold: One, sausage was not a main ingredient on pizzas in the east as it was in the midwest. There was no big tradition of sausage on pizzas back east (although I hear that Boston may be an exception, as well as a couple of the famous NY/NJ pizzerias). In Chicago, sausage was and still is the Number One ingredient requested on pizzas and number two nationwide. And in Chicago, it is almost universally put on the pizza in a raw form. Outside of Chicago, pepperoni is number one.
And the second and most important reason for sliced sausage being used is that most east coast style pizza is a hot and fast baked pizza generally. Hunks and clumps of sausage, which is traditional to most mid-westerners, would not cook thoroughly with such a fast cooking time. But with slices, it doesn't matter. Slices, even if cooked previously just a little bit, will cook quickly in a sliced form. In Chicago and a lot of midwest places, pizzas are commonly cooked at a little lower temperature and for a much longer period of time. In cooking for such a longer time, the raw pieces or hunks of sausage gets cooked very well. I much prefer the hunks or balls of great, tasty Italian sausage on pizza, rather than slices, but I guess its all what you are used to or raised on. Down here in Florida, most of the pizza places are New York/NJ or east coast style and serve sliced sausage on pizzas. A few of the Chicago, St. Louis, and midwest style pizzerias do it the right way though, at least to me. But I've come to like it both ways.
An interesting article about sausage on pizza is at http://pizzatoday.com/production_articles.shtml?article=Nzg2MHN1cGVyNzg1N3NlY3JldDc4NjQ=