In terms of menace potential, any private plane will lose to a van full of fertilizer and a baddie intent on causing destruction. It's a matter of scale.
Little planes, like this one [1] just don't do damage on the same scale as airliners.
[1] https://en.wikipedia.org/wiki/2010_Austin_suicide_attack
Government building codes already anticipate the "van full of fertilizer" attack, as a result of the Oklahoma City bombing in 1995. Federal building security is a separate matter though, with its own agency called FPS that predates DHS and TSA by decades.
What about a private plane full of anfo
Most private planes taking off from commercial airports (the ones where TSA generally operates) are much larger than a Piper Dakota.
(But regardless, it’s not clear that the TSA is even performing that kind of calculus.)