Bitcoins are the market's response to credit cards banning certain companies or industries. Even in states where a certain "substance" is legal, you can't
buy it with credit cards as it then involves federal banking. The same is true for other "gray" areas- private file sharing sites, newsgroup readers, etc. Bitcoins are as anonymous as possible and secure, provided you use a reputable vendor for them.
You can't buy bitcoins with a credit card, at least not directly. You have to wire the money to your bitcoin vendor, who then adds the credit to your account. At least that's how I had to do it.