It is the responsibility of the applicant to prove conclusively who they are and whether they are a threat to the host nation they want to travel to by whatever standards the host nation deems fit. The host nation has absolutely no obligation to let any applicant in. Personally, I would like to see them add a few more questions to the application:
1) How are you going to support yourself while visiting?
2) If allowed in, are you going to bring anybody else with you later down the road?
3) If allowed in, how do you plan on assimilating into American society and culture?