Certainly does make sense to have a 3-6 month probationary period for a permanent contract. Whether experienced or not, I think this is a reasonable practice that in a permanent contract would only be invoked if absolutely necessary.
However, I think the issue here is the cynical use of a year-long "probationary period" attached to a non-permanent, flexible contract. And allegedly having to pass another interview after that year in order to keep your job. My worry would be what happens if, after the year, you find yourself surplus to requirements without the protection of an employment contract?
Given that the application was for "First Officer", I'd say the goalposts were most definitely shifted when they decided to offer flexicrew cadet status to experienced applicants. I can't help but wonder how many will be willing to relegate themselves back to the very beginning and accept more debt (well, I don't have £20k sitting spare anyway) for the sake of future potential prospects.