Yea that was a concern and a choice we made. It would indeed be completely trivial to follow people who tweet at the bot.
In the end there were a couple primary reasons I think we decided not to do that.
First: Nobody really uses direct messages. I don't think I've ever received or sent even one.
Second: The idea is for it to be trivial, little things. Things that would be meaningless if seen by someone else. Eg, not "colonoscopy ; tomorrow 3pm", but more mundane stuff like you know you're going shopping tomorrow so you ask it "don't forget to buy milk ; tomorrow at noon". I check twitter randomly throughout the day, so these sorts of jog-my-memory things are useful.
Lastly: We thought that perhaps if it gains any traction a direct message option could be something available to premium account holders, or something :)
Definitely don't think of it as a replacement for an alarm on your phone or something, it's about casual stuff.
"@lmklater check on lmklater post on hn ; in 3 days" :)
Wouldn't DM work just as simply? Granted you'd have to follow every single signup but that can be automated.