Hehe, kind of ironically that they allow donations with Credit Card.
An anti debt movement basically allows you to donate to them with debt.
Well, I'm not a American, so I wouldn't donate to them even if they would accept BTC donations.
And as I find it wrong to encourage anyone to accept BTC Donation without actually planning to donate if they do I wont contact them. But the contact field is right there:
http://www.thecankicksback.org/contactIt costs you 1 minute to write a little text and inform them about BTC (and maybe BitPay if they only want USD).
So just do it.