Why would a former employer say negative things about a former employee or would want to do so. If the employee did a poor job or engaged in misconduct, then they may wish to prevent another employer from making the same mistake.
You could have a lawsuit if they say something bad and it keeps a former employee from getting a job? Not if the information provided was factually truthful or represented their honest opinion.
Just because there were issues at an old job, does not mean there would be at a new job. Depends on the circumstances. If the employee and the job were mismatched, I agree with you. If the employee was frequently absent, negative, unpleasant to coworkers and so on, that's going to go with the individual to their next job.
You might just have been a bad match somewhere but be the best employee at the next job. That person no longer works for you so why would want to ruin their chances for the future? See above. If the person engaged in some sort of misconduct, they've hurt their own chances for the future, not the employer.