Well, if you fly once in 5 years, then a single scan/trip through the queue is probably no big deal in terms of time, potential radiation exposure, etc. So don't spend the money. No sweat off my behind. And frankly, if you fly once every 5 years, you probably have other things to think about than whether or not Pre-Check is "fair" or not. If you fly once in every 5 years and have strong feelings about Pre-Check being available for a fee, then you need to reconsider what you'r choosing to get upset about. There are bigger issues.
If you fly every couple of years and the extra $30 bucks is going to be too much of a burden, then wait in line with your too-big-to-cram-into-overhead suitcase. Again -- who cares. It's your choice. Opt out if you don't want to be scanned. Nobody is judging you if you want to fly cheap. Why judge those who want to spend a little more? It's their choice.
As for the marginal cost bit - if you can afford to fly domestically to get from point A to point B, then you're already demonstrating that you've made a judgement that your time is worth a certain amount, because there are cheaper ways to get from point A to point B in the US (Amtrak, Grayhound, walk, drive, bike, hitchhike) that don't involve flying. People fly because these modes take longer. The truth is that flying is not a 1% activity anymore. It hasn't been for years. There are over 600 million boarded passengers in the US each year. Even if the 75% of those flights were taken by international travelers, business travelers, the wealthy elite, and frequent fliers, that's 75 million passengers per year in a country of 300 million. Clearly there aren't 75 million millionaires in the US, so I'm betting that flying is a basic mode of transportation accessible to most people. And since the average domestic US airfare (regardless of point of origination) is between $300-500, then the fact is that even the full $85 for the 5 years of PreCheck is to some extent a marginal cost (especially when you consider travel to-from airports, parking, etc.).
I'm not projecting my personal situation on the rest of the country. I'm just making a rational and objective assessment of the cost of PreCheck versus the cost of travel and judging it to be marginal. Those are the numbers.
So are you that upset that people choose to spend a little bit more to save a little more time? Are you pissed that government makes it possible? Are you bitter that some people choose to pay more for something that's a little "better" (faster/easier) in their eyes? Do you get this upset at people who pay the extra $65 for expedited passport service? Or those who pay the extra fee to upgrade to first class on the airplane? Or those who choose to pay $3.00 for a Starbucks instead of $2.00 for a Dunkin' Donuts? I don't understand what the complaining is about.