A perfectly valid question and one a couple of folks have asked so I'll answer.

There are two reasons... firstly because the graphs are not private I didn't really see a need to make the tweets private. We could add the complexity of making you create an account and hiding your data away but part of the idea behind gtFtr is to encourage folks to work out and get better - I put my stats out there for folks to laugh at my pathetic pushups and make me want to do better. For something like SugarStats (a great tracker for diabetics - I'd actually thought about adding the same metrics but they do a great job so I won't) I can understand why folks would want to keep it private. So firstly it's a motivator.

The second reason is that it's social and viral. I want your friends to see you sending tweets to gtFtr and get curious and come and check it out. Web 2.0 is all about viral growth, Twitter is all about communications and so it's a great way to get the word out there.

If enough folks want me to look at adding direct messages I'm certainly not going to say no (though it adds complexity so it might not be top of the list, especially if we need to get into making the graphs private as well) ... use the comments and let me know what you think....