They have to be getting some income, and if it isn't through welfare fraud, I suspect it is from apparently legitimate Islamic charities that funnel money to them. What these terrorist cells do is they take advantage of these charity groups by planting members in them. Unfortunately, even though some of these charities probably do some good, they need to be audited at a minimum, although I prefer the harsher measure of freezing their accounts if the slightest funneling of money to troublemakers is detected.
Regarding the militarization of the US borders, it is already happening. We will soon have a helicopter air base in Plattsburgh patrolling the border and roads going to and from Montreal. Depending on what happens in the future, I think full militarization of both borders will eventually occur.