I disagree. As I stated above, refusal to sign the form IS reason enough for them to fire you. As LS16SpdZ99 correctly points out, you can be fired for no reason whatsoever in Wisconsin! If you are refusing to sign their form you are basically refusing to follow the employer's work rules and that IS a legitimate reason to fire you.
Employers cannot fire someone for a discriminatory reason, but any OTHER reason is legal. Firing without a reason is legal. A "discriminatory" reason means one of the few things specifically outlined by law, e.g., because of your age (over 40), your gender, marital status, religion, color, etc.
Now, if one-half of the employees refused to sign the form and they only fired, say, the black workers who refused to sign it, or only the older workers, you might have a viable discrimination complaint. Lacking that, you're screwed my friend. And that's exactly what they'll tell you when you call the state Equal Rights Division. Call them and ask.