The C-17 has a unique reverser system, it is a special kind of system only for the C-17. Since the aircraft is designed to takeoff and land on almost any runway, including unpaved runways, and its a turbofan powered airlifter, if you had conventional reversers found on airliners, you will be kicking up a heck of alot of dirt and that'll be sucked back into the engine and cause millions of dollars in damage. You wouldn't want to pay that kind of bill wouldn't you?
So the reverser system is unique in that when it is engaged, the exhaust is thrusted forward and upwards away from the plane, in other words, no FOD damage. If you study the picture carefully, you can see that the cascaders are blocked underneath but open above...the exhaust is thrust forward and upwards.
With this system, you can keep the engines running and engage the idle reverser system and just immediately begin to load or unload without having to shut down and restart the engine. The C-17 was designed for missions where you land at an airfield, immediately unload and/or load troops and cargo and takeoff immediately. Jet engines take a while to start up and get them up to speed. Its basically a time saving measure and it also prevents your troopers and cargo from being blown all over the ramp.
Thats why you saw the reversers engaged when the C-17 was parked, its only to prevent jetblast from blowing everything around and you can offload off the aircraft as soon as it stops, instead of having to wait for the engine to spool down before its safe to access the plane.
Airliners on the other hand, they aren't designed to land on unpaved runways, only paved runways and they're usually free of debris, so this kind of reverser system is not quite necessary. Jet engines can only take a certain amount of punishment from FOD before they conk out, but what are the chances of having a really dirty runway at a major airport?
That answers your question, probably more than enough.
Jason @ CYVR