If someone plans to go back in time, and doesn't plan to be interrupted, they won't normally get the message because they weren't trying to find a loophole. If someone else decides to send a message further back by standing in front of where they were and going back in time, then how will that message change anything? I suppose it could send you the message before you even think about that, but what if it's spur of the moment? Will it send you a message just because you were going to do something? Then again, it's not as if those messages can't be used to get information. Dumbledore totally did that when he found that message saying "no".