it won't just "fall" into the earth's atmosphere. obviously you didn't read the article, it's a disposable vehicle, whatever main engine fuel that isn't used for space missions is used for accelerated re-entry and destroyed with the vehicle and the garbage it contains. that fuel could be used to set a trajectory toward the sun instead.
The ATV is currently moving at about 8 km/s. The escape velocity of the Earth is about 11 km/s. So you'd need enough fuel to increase the ATV's velocity by about 3 km/s to get it out of Earth's gravitational field. And then you'd need some more fuel to put it on a path which a few years later would take it into the Sun. By contrast, you need to decrease it's velocity by about 70 m/s for it to reenter a few hours later.
And if you were really, really determined to chuck it into the Sun, you'd need to launch another rocket carrying an upper stage, which would then rendezvous with the ATV and take it away.
dont forget there are 2 planets in between the earth&the sun.
and if you send send it at the sun, its likely it could slam into one of those 2 planets instead. and we dont want to start littering other planets.
so boosters would be needed to steer it to the sun, a trip from earth to the sun is ALOT longer than a trip from earth to the moon. it would need constant supervision as well on its trip to make sure it didnt get sucked into venus or mercurys gravitational pull on its journey. which would take up extra manpower to supervise it.
or, you can send it back to earth, destroy it here, or maybe even recycle parts.
which sounds like the easiest of the two to do?
Space is a very, very, very, very big place, and planets are very, very small compared to it
The chances of you smacking into another planet are probably near to zero, if you left orbit at a random time. Also, once you'd set it on it's final course, you wouldn't need to keep an eye on it at all - the Sun's gravity will do that for you. Not much of the ATV will survive to the Earth's surface - there's one part with some instruments that will hit the ocean, but that'll just sink. Same goes for the thrusters, since they're meant to withstand high temperatures anyway. So anything that does survive will be in an unknown location at the bottom of the Pacific...not exactly the easiest stuff to find