If you have flown 2 sectors and are then asked to do a SBY then it must be as a continuous duty and having already operated, your FDP, not DP, continues and all the while, your maximum allowable FDP is ticking away until you reach the time when you can't go anywhere anyway because you'll be out of hours if you did.
Your subsequent rest period then needs to be based on your total duty, from report time until such time as you go off duty (after SBY) and this rest period would not commence until having finished the period of SBY.
Yes, this is legal and is normally associated with smaller operators, where profit margins may be tight thus in everyone's interest, to keep themselves employed, to be available should an adhoc flight or charter materialise.
No real need for such an outburst as above without knowing more about particular circumstances.