They were expecting military plane, they were told THIS plane is military plane and they shot it down. And after that, they were very surprised to find sea of bodies, not military equipment.
If you have a equipment to down planes, it's also your duty to have the proper radar equipment to reliably see what's up there, something that those Rebels did not have. They just stole equipment out of their league, did not know how to handle it and killed 300 innocent travelers.
On October 4, 2001 they accidentally shot down Siberia Airlines Flight 1812, killing 78.[1]
For what it's worth, I do agree with your sentiment.
[1]: https://en.wikipedia.org/wiki/Siberia_Airlines_Flight_1812
As an aside, the IR655 tragedy was one of the first were data was available to separate fact from fiction.
http://www.nytimes.com/1988/11/18/opinion/witness-to-iran-fl...
This tragedy, IR655, and KAL007 are all stark reminders that war is often without discretion.
One of: http://i.imgur.com/4XgZCaH.jpg
Just "oh I wonder what this button does"?
As you say, they just got their hands on equipment and had little clue how to handle it. They are also fully responsible for what happened.
If you plan to use such weaponry you have a duty to confirm whatever it is your are shooting at is the right thing. Especially if it is a known flight path for commercial airliners.
I agree with you that they likely didn't intend to shoot down a commercial, rather than military, plane but that doesn't absolve them of responsibility.