Most events producing conspiracy theories have important social and political implications, and shooting down a commercial airliner (presumably by accident) certainly qualifies. Anti-Obama commenters have used the event as an opportunity to revisit real and perceived grievances and cover-ups, including the Benghazi attack, Edward Snowden's spying disclosures, the IRS targeting groups for audits and much more.
Floating wild conspiracy theories without a shred of evidence is a ghoulish parlor game for many people, especially online. After nearly every tragedy -- from the September 1, 2001 terrorist attacks to Princess Diana's death -- black humor soon surfaces in the form of tasteless jokes, and these theories aren't much different.
Even those who circulate these rumors surely don't believe all or even most of them; after all, the destruction of the plane cannot have been the work of the United States and Russia and Israel.
Still, research has shown that conspiracy theorists have no trouble holding onto completely contradictory beliefs -- so they don't let the facts get in the way of a good story.