Yah, as I posted in the thread on the issue about having to kill apps to save memory, a LOT of developers are most likely younger kids in school, learning Java, and don't have much experience/knowledge with all the ins and outs of app development, especially for a phone. Even if they have Exit/Quit, they may not properly release resources causing the app to hang around until you kill it or Android does to regain resources. For those of you that know much about the old C(C++)/Java wars (why one is better than the other), there used to be this notion that Java doesn't have pointers (one of the greatest things about C/C++ but also the easiest to cause problems with programs, crashes, etc). Fact is it does, and this is why a lot of Java apps don't exit properly... too many java developers don't realize that they do have to release various types of resources or the JVM will keep the app going.
Hopefully that will change as more senior developers start writing apps for Android.