Do you 100% know they are?
Why would the PRL take on Saracens, poster boys of English rugby, supplier of a large number of internationals, only English club with any success in Europe in recent years, with the weight of an incredibly wealthy financiers legal team behind them - while simultaneously letting everyone else get away with it?
They're the last club you'd pick a fight with.
I mean Wasps were caught (for a minor breach) last year, so it's not like everyone else isn't being looked at.
As Tigs Man (I think?) mentioned above it seems like a number of clubs were caught in 2015, but everyone agreed to keep it on the downlow and move on - Sarries kept on with it in a blatant way so have felt the full weight of disciplinary action.