Publishing PSDs of someone else's website—regardless of who made the PSD—is a dick move. Doubly so given that the authors resources contain the original companies branding and image resources ripped right from the original pages.
There's nothing in your little agreements—or lack of one with MailChimp—giving you the right to parade around as if they have harmed you.
That is Allan's right, and Heroku's right. But Allan can only speak for himself, Heroku can only speak for Heroku. So while it's good that you've asked them for their opinion, they have no agency and confer no expectations as to what MailChimp does or does not do.
You're not really giving people a choice if you expect only one answer when you ask them for permission, you know. ;)
But you need to decide whether you're asking permission or getting offended. I don't think you can have it both ways.
edit: you do misattribute them (e.g. "Facebook Timeline UI by Luke Chesser" https://github.com/lukechesser/Popular-UIs/ )