Because people like me, when they enter a message, will sit here and click over and over until they see their own message. And with the number of flood posts etc. you never get to see your message because it's buried in the crap, which is frustrating.
You can get around it by firstly removing any message that isn't unique, and it might also be a good idea to have a timeout where you don't accept another submission.
Clearing the input box between submissions would stop people from just clicking the share button multiple times too, then they'd at least have to paste the text in before clicking submit again.
I've filtered dups and cleaned up the submit process so it makes it a little harder to spam, but I guess it just seems silly to me to try and actually stop spam on the internet.