Yes it is unfair. And there are exceptions to discrimination laws for certain cases eg. for small companies, or time critical operations - similarly to exemptions for jury service.

If you make it clear in the interview that there is a deadline - we are working to a product launch in 6months and we need you to start immediately, and you took the job without telling them that you were immediately planning to take 6months off then the employer would have a pretty good case in most jurisdictions.