Come on mate,this is hardly a valid point. Remember Headly came on a VALID VISA but was unable to raise suspicion as he was a US citizen with a Christian sounding name. These kind of cases are going to be hard to prevent, these guys aren't stupid and whatever the rules on the ground they will work to find away around it. The only real way of preventing such attacks is a very robust intelligence effort against these guys, US allows Pakistanis/Saudis/Somalis/Yemanis in but they have had no terror attacks since 9/11 because of the security apparatus now in place. The real issue is the potential economic impact the Pakistani migrants could have (like I said US-Mexico analogy) as India is only going to grow and grow and become an even more attractive looking workplace for illegal Pakistani workers.