Actually, it is about confusion. Most trademark infringement laws around the world have a requirement for "confusing similarity". In the US this is known as the "likelihood of confusion test".
http://en.wikipedia.org/wiki/United_States_trademark_law#Infringement:_likelihood_of_confusion_test
A very good example of this was actually legal action against Apple in Apple Corps v Apple Computer.
Apple Corps is a multimedia company created by the Beatles. They sued Apple Computer over trademark infringement numerous times. An original settlement stipulated that Apple Computer could continue to use their name as long as they weren't involved in the primary industry that Apple Corps was (ie. Music) as they would likely result in confusion.
They were later sued for breaking this condition once they created the iPod and iTunes.
http://en.wikipedia.org/wiki/Apple_Corps_v_Apple_Computer
I wonder if it mattered there that "Apple" didn't exactly describe the computer. App Store just defines what is sold there. Apple is just large enough to claim acquired ubiquity. Also of course they were sued for breaking their agreement.