You needed visas to enter the US well before 9/11. And the visa waiver program was always restricted to B-1/2 visas. Sure, they may have become stricter, but fundamentally 9/11 didn't change all that much as far as visas is concerned.
Incorrect, prior to 9-11 and patriot act there were no restrictions on people that qualified for a standard tourists visa walking into any flight school with an ID from there home country and training for and receiving a PPL, post 9-11 when the Feds realized how the hijackers had got there training new laws were put in place that put rules on both the trainee and flight schools. Just try and get a FAA DE to enter the approvals to take a checkride into the FAA online system without him/her checking for the correct school and visa paperwork and see how far you get.