This is completely incorrect, and it's a little insulting to the users here who aren't American.
Freedom of speech is a value with broad applicability. The United States Constitution enshrines a right to free speech as a limitation of government power, fine, but they don't have a monopoly on the idea. Relative freedom of speech is a characteristic of every forum in which speech and the suppression of speech are possible.
Banning users of a forum limits freedom of speech, removing commentary that goes off-topic limits freedom of speech. Forums are often quite free to set their own rules about who is allowed to say what, and users are often free to choose the forums they frequent, but that's mostly beside the point - speech within any particular forum is free to a given extent, and some people (be they owners, operators, users, whatever) believe that this freedom is intrinsically valuable.
(I don't really see this one as a free speech issue myself - to me, this one is about the credit card company preserving their brand integrity.)