Weeeeelllllll... that's the starting point for an interesting discussion, although it would rapidly get OT. Best stats I can find indicate that about 1 in 120 adult Americans is currently in jail, 1 in 32 is currently in the Justice system (including probation and parole), 1 in 12 has been imprisoned over a lifetime, and at least 1 in 4 has been arrested.

However, air travelers are not randomly selected from the population. For example, 100,000-mile-per-year flyers are quite rare in the population, but there are probably several of them on your next flight. I suspect that the formerly imprisoned are relatively rare on airplanes, for obvious socioeconomic reasons.

That said, while I'm skeptical that you can be "assured" that a convicted criminal is on any given flight (and intrigued by the math problem), I agree with the main point, which is that it occurs with some frequency.