The sign up/don't sign up button is dynamically generated based on the perceived desirability of the user. I wouldn't take rejection badly, the main thing is to learn from it.
How did they determine the "perceived desirability of the user"? Seems a little presumptious to me - I didn't even know what it was until I clicked the link, and the first thing they say is "we don't want you."
I took it to mean "instead of buying, try it out."
After hearing the part about the "perceived desirability of the user", I'm thinking maybe they only want to sell to certain people today, perhaps return visitors? Maybe they want all their new visitors to try it out before buying? They could have the button change based on cookies, and they just started an ad campaign that's on HN's front page.
Edit: This comment was unhelpful? If not, why downvote?
