Why 80%? I've worked in various companies with 3 months probation (actually, quite common in my country) and it was fine, but never for a % of the salary (unless it included % less time at the office)
It's scummy.
You could simply hire better employees rather than disrespecting your candidates. I wouldn't bother pretending that it has anything to do with "risk management". At best, you sound extremely cheap. At worst, it makes you sound like you repeatedly make poor hiring decisions and scramble to adjust for this.