Roger Long wrote:

Not being able to train in a normal flight school will not prevent the
determined terrorist from getting enough stick and rudder skills to carry
out an attack. There are plenty of drug runner pilots and similar types who
would teach him for a fee and out of anyone's view.

And for them it wouldn't even be illegal. The rules only apply to people
trying for legitimate ratings with legitimate flight instructors. BillyBob,
just a private pilot, with no CFI can give the instruction to Osoma with no
obligation to the TSA or anybody else...just can't sign him off for his
checkride.