I'm sure that kids do get overheated at WDW almost every day when it's hot. But that's not always just due to strollers. And just because you see children in strollers does not mean that they're spending their whole day in them.
I'm very into attachment parenting and believe that babies should be held when possible, rather than placed in assorted baby-holding objects, but a stroller is not always evil. For some families, it is the best option some of the time. Sometimes parents need their hands free to do things. Sometimes parents simply can't hold a child for an extended period of time. "Average" babies may be easy to hold for most of the first year, but a lot of babies are over "average." (Exactly 50%, as a matter of fact.) My 6 month old baby is the size of an "average" 2 year old. At 5'2", I have to put him down sometimes.
It is very important to keep babies cool, but there are lots of ways to do this. The best options include simply keeping them out of sun and crowds during the hottest parts of the days. When it's necessary to be out when it is hot and crowded, battery operated fans, squirt bottles with water, and appropriate clothing are all good ways to help keep baby cool. Of course, no baby should routinely spend lots of time in a stroller if the parents can hold the baby, but it's more important because babies need to be held than because it's a way to keep them cool.