They would only be liable to potentially pay IF the employer was at fault in causing your fall and injury, since liabilty (or paying for another's medical costs) is based on fault. If they were not at fault in your fall--e.g. you didn't fall due to some unsafe condition at work--then if you accepted the ambulence ride (you could have refused it) and therefore received the service, you will have to pay for it.Note that even if the employer should be liable (i.e. if it was their fault), if they refuse to voluntarily pa, you'll have to have to pay EMS yourself then sue the employer for reimbursement and prove their fault in court.