the BUK system is equipped with a civilian transponder safety lock which has to be manually disabled before a missile can be fired at an aircraft showing a civilian IFF.
One plausible (but possibly wrong) explanation I heard was that the radar on the launcher vehicle is really just turned on for targeting after proper identification, and that the search radars normally on the other vehicles are used for identification and have the transponder detection bits.
They must spread em out like that to lessen the chances of the launcher being detected and targeted.
The rebels (and their Russian handlers?) might only have had the launcher vehicle to work with. The Ukrainian surveillance photos I've seen only seem to show the launcher. Of course that doesn't rule out the other vehicles being there too.
But if the other radars were actually present, then it really does look deliberate or at least incredibly incompetent for them to think it was a military plane.