Based on what you write, this was illegal: allowing you to use a different kind of seat (ergonomic chair or stability ball) for a provable back condition--especially when you would provide them and there is no cost to the company--is illegal disability-based harassment or discrimination. Employers must make reasonable accommodations to employee medical conditions, and what you describe is reasonable. Their alleged liability issue could have been dealt with easily: you could have signed a waiver that you would sue if hurt by the chair or ball you provided; therefore, it was a non-issue.You should contact the federal Equal Employment Opportunity Commissino about filing a complaint; you may be entitled to compensation. Good luck.