Good question, Mashood. I doubt the separatists have the same level of familiarity with the Buk/SA-17 system that the Russian and Ukrainian armies have, or that they have established safeguard protocols in place like most professional armed forces do. The separatists are at war with the Kiev government and want to shoot down Ukrainian aircraft to further demoralize the Ukrainian armed forces and reduce their will to fight.
The Russians don’t just hand over sophisticated weapon systems, then walk away. They provide “advisors”, whether military or civilian to assure that the systems are being used properly and are functioning properly...the information and data is accurate... Etc... Same as the US.