I totally understand where Bertie and Rory are coming from.
If we're to believe the reports that Saracens have been doing similar deals for years with approval from PRL /salary cap manager, then why would this current crop of players believe that it's suddenly no longer legal?
That being said, we only know of deals for a handful of their players - their absolute top stars.
Even if it's all out in the open at Saracens so that everyone knows it's going on (and that they have parties where they laugh at the rest of us for our naivety), it would still be a hard push to believe that their wider squad players (Malins, singleton Loz, Barrington, Kpoku, Isiekwe etc) know enough about employment law to question what's going on.
Even if they ask their advisors, (agents / accountants /lawyers etc) the deals sound similar enough to those that have previously been approved that you'd have to be an expert in the field and have full details to hand to stand a chance of categorically knowing to exact legality.