Well, you can already pay extra to expedite your passport application. I've paid the DMV a couple extra dollars to renew online instead of in person or via mail. It's equal service to those who pay for it.
By the same token, why not have an "Oh, crap!" kiosk for passengers running late? They can pay an extra 20 bucks and get a pass that allows them to use a shorter line. Maybe airlines could include it with first class tickets (you certainly pay enough for them, it's not like you'd notice the extra $20). And if you don't want to pay, you use the longer line your taxes bought. If they price it right, they could hire more people for both lines and everyone wins.
Not sure how airports in the US work, but aren't first class tickets already supposed to give you a shorter line? I am pretty sure the last time I booked a flight (extra-low economy class - easyjet) there was an option to pay for a shorter line.
You can buy your way out of the line at the gate (first class boards first, despite the fact that this is decidedly non-optimal from an operational perspective), but the security checkpoint is not at the gate. The gate line is managed by the airline and is primarily concerned with ensuring passengers get on the right plane. (They saved me from an impromptu trip to Seattle when I was to the JavaOne conference. International travel does strange things to the brain.) Another concern is earning the airline money through rigidly enforcing the class system. (+)
The security line is managed by the TSA (an arm of the government) and its (stated) primary purposes are to ensure bad people and bad things do not get passed it.
(+) I watched the 2nd to last person onto the plane get yelled at for going through the left priority boarding line when she was holding an economy ticket, and holding an infant to boot. They made her back up three feet and go through the economy boarding line. It someone could have telepathically read my thoughts they would have probably taken away my Vast Right Wing Conspiracy card.
Southwest Airlines has priority lanes at the security checkpoint in several airports for their Business Select and A-List customers. You get to move to the front of the screening line if you pay for those tickets. This is exactly what Clear was doing without the stupid background check.
I'm for government adopting the same best practices as would private firms performing similar operations. When all the post-9/11 distortions finally work through the system, we might go back to a regime where many/most airports handled their own security, with private firms, but to federal standards.
How does funneling pilots through the screening line do anything to prevent non-pilots from dressing as pilots? Real pilots ride as passengers, in full uniform, all the time.
I expect the reason is even simpler than you suggest; a total lack of exceptions to the screeing rules makes for simpler screening, simpler training, fewer chances to make mistakes, etc. And it's easy.
It doesn't prevent non-pilots from dressing as pilots. It just means (ostensibly) that even if you can pass as a pilot, you can't bring shampoo on the plane.
And I'm sure dropping the background check would actually make the service more attractive to people.
I'm sure that there is tons of personal info that could be found by someone motivated enough, but I'm still not comfortable giving out detailed information on myself. So I would be hesitant to use a service that required a background check, especially if I don't think the information is needed.
And not requiring a background check would have let them be more flexible. For example, they could have started selling at the airport one-time use of their lines.
But the fact remains that it doesn't make much logical sense as a requirement (to do background checks of people who go thru the same checkpoint as those who don't get background checks); if they were forced into a bad business plan by the government, they were still silly enough to stick with a bad business plan and pray that one day it would suddenly start to work.
As I mentioned in a comment above, Southwest currently has a process in place in several airports that allows you to cut in the security line sans background check.
A simpler version of 'Clear' -- just paying to 'cut' in line -- could be run on 'Paris Metro Pricing', as described in this paper about network routing:
Specifically, make two lines. They are identical except one costs a small fee. As long as people can accurately observe queueing times, they'll choose appropriately to get a value in time saved greater than the out-of-pocket costs.
What kind of background check can you get for $200? I doubt they could find out anything less obvious than past convictions or frequent visits to countries associated with terrorism.
I'm perfectly fine with paying more to the airline to cut to the front of their line. But everyone should get equal service from the government.