[1rudeboy]

The one thing I've never understood is why the Russian rebels figured that something flying at 30,000+ feet was a threat.

My guess is that they just wanted to play with their new "toy." Probably drunk.

[some tech guy]

As I remember it, they thought it was an AN-26 which is capable of FL250 or so but no higher.

Here’s the intercept transcript release by Ukraine: http://www.ibtimes.com/mh17-crash-full-transcript-alleged-phone-intercepts-between-russian-intelligence-1631992

Thing is, MH17 would have been a lot higher that an AN-26 is capable of, plus it had its blasted transponder on. There’s no excuse.

[annalex]

why the Russian rebels figured [...] Probably drunk

You answered your own question. They did not figure much. After watching a few videos of these "rebels" one gets the idea that those people are not strong at figuring anything. Anyone who gave them anything more powerful than bows and arrows committed a crime against humanity.