There is a form in Missouri that several employers use to find out your pre-existing health conditions or procedures that the prospective employee has had. The form states that if the employee doesn't disclose everything it could be grounds for dismissal so of course the applicant discloses and the employer doesn't hire them. Is this legal?

If they are using the knowledge of any pre-existing conditions as a way to weed out applicants, no, that is not legal.

However, I think it might be difficult for any given applicant to show that the specific reason THEY were not hired was due to this form. After all, out of all the applicants, only one is going to be hired.

Why would they be allowed to request this form to be filled out in the first place? I'm from California, we'd never in a million years get away with that.

State law varies, and CA law is wildly different in many regards from that of the rest of the country.

However, I didn't say they WERE "allowed" to ask for this information. I said that it would be difficult for an employee to prove that they were turned down SOLELY because of the information provided.