You're misunderstanding the difference between the rules of the network and the actions of the participants in the network.  If you send a message over the network then listeners will use the rules of the network to parse your message.  If they can parse it, it fits the rules of the network.  What your peers do in response to your message is in part determined by the rules of the network, because if your peers send unparseable messages no one will understand.  But even within the rules of the network, your peers have choices: they can rebroadcast your transaction to their peers, they can turn off their machine and go home, etc.  The rules of the network is not the same thing as the actions of the participants.