I agree with you about it being a conspiracy theory, but going by the history of Poland where the Russians have been playing intrigues since the 18th century, it's a bit easier to understand why it's to be believed
Also note that the Russians were pretty slow to get to explanations, the investigation was haphazard etc -- breeding grounds for a conspiracy
That the Russians caused the accident is doubtful. -- improbable, yes, but going by Russian intrigues, not impossible. And that's the catch -- in Poland it's easy for Poles to believe the worst about the Russian government
All true, there were screwups before and after the crash during the Russian led investigation. Nevertheless, you should have heard some details of those conspiracy theories - the Russians created artificial fog, was one, another one had the Russkies killing the survivors with shots to the head. The conspiracy is especially popular among the super patriotic countryside segment, listenership of the Catholic radio and TV Radio Maria.
But the cause of the crash appear to have been rush to get there, incompetence on both sides, and bad luck.