It is only an instant heal if it heals instantly, and PoM doesn't. What you are trying to say is that it is an instant cast. An instant heal has two benefits, you can cast it on the move, and the heal can't be interrupted. PoM is a buff on the target, and buffs can be countered unlike instant heals.
As for the duration of Sap, it is pretty damn long but the Rogue simply stealths and waits for the buffs to run low and Sap before they are reapplied. The difference here is that the PoM buff already can be countered whereas a true instant heal can't. Sap was just only one example.