Since this thread is continuing, I'll point out that randomness doesn't matter.

A random inspection is useful if there is persistent activity. Like speeding. Or continuing embezzlement. If you regularly do such an activity, random enforcement will deter or eventually catch you.

But terrorism isn't like that. It's like a single bank robbery rather than slipping a few hundred dollars into your pocket every day. A 90% chance of success is pretty good odds for a one-off attempt.

So what exactly does random selection for screening accomplish?