Quotas for traffic tickets, long denied by police but generally accepted as fact by drivers, are pretty bad. Speed limits are a revenue measure more than anything else. But the same principle, applied to air marshals and the terrorist watch list, is much much worse.
The simple truth is that there aren't enough terrorists for thousands of air marshals to identify one "suspicious act" a month, even if the only thing the terrorists were trying to do was fly around the US to scout out our air security. Most likely the terrorists aren't even bothering; they are looking for other methods of attack while we focus on costly security theater.
The only reason for this kind of quota is a beaurocratic exercise in justification. You see, if the terrorists aren't probing the airlines, we might decide to shift resources elsewhere and use less costly methods (like, say, armed pilots) to defend air travel.