I disagree that to have a uniform standard for searches that the agents performing the search must be government employees. There are several major airports that go fed up with the poor quality work done by federal employees, and with the complaints from passengers on the federal employees. What they did was hire their own security that had to meet the standards set by the TSA. People were happy as complaints were met with greater interest and care, those being rude or not doing their job were fired or moved away from the passengers. There is still a TSA presence at these airports but they act as inspectors and supervisors, never performing searches themselves.
As to your second question I never did have my colon searched but I have been given additional searches at just about every airport I visit. Might have something to do with my appearance. I'm a pale white farm boy, 240 pounds, 6 foot 5 inches tall, crew cut hair (slightly grayed), eyeglasses, business casual dress, and since I despise checking in luggage I tend to carry two bags on board with me. With my bad feet I tend to carry a cane. And considering that the flights are typically cool I'll carry a long duster style coat. Considering my business I might have a magazine on guns, computers, military, hunting, or flying. Carry on bag may contain a computer, multimeter, safety glasses, gloves, hearing protection, and steel toed boots.
Someone inevitably asks me to step aside so they can poke around through my bags and ask me about where I'm headed, what I plan to do there. I'll talk about high voltage oscillators, fast running pigs, airplanes, and shooting things.
Perhaps they find me interesting. Perhaps they want to make double sure I have nothing sharp on me. Perhaps they want to keep me aware from their women and children are safe from someone so dangerous. I can only guess they have heard of my motion sickness. I drug myself as I board the plane.
I don't know what they look for, whatever its they don' find it.