It should've been unchecked by default, because that's what a majority of new sign-ups would expect/want. Instead you could ask for a promotional tweet on users' behalf once they become established users (by some reasonable metric).

A. Oh, I wish I had seen and checked the box, that initial tweet didn't get sent out.

B. Oh, I wish I had seen and unchecked the box, I was surprised that a tweet got sent out.

Which way do you think the vote would go if you asked people their preferred scenario?

I don't disagree, and at least there was a checkbox, but blaming the user for what's at best a UX issue and at worst an abuse of trust doesn't bode well for a new service.

Moreover, they're asking users to make a decision about whether to promote their service without first using it. I think you'd get more engagement if you populated your timeline and there was a message that said, "Hey, isn't this cool? Let us know what you think, or tell your friends about it. Thanks for using <SERVICE>!"