When any command that allows multiple targets fails, for each entity it picks up, it spams the chat with the usage info. For instance, if you were to do ' /entitydata @e ', for each entity loaded, you would get the message ' Usage: /entitydata <entity> <dataTag> '. This floods the chat pretty quickly.
you select to execute on every entity, each entity sees a incomplete command and replies back with the help syntax, works as intended