The charedim generally take more in benefits than they pay in taxes, since most dont work and 56% of them are in poverty, they get social benefits.
The can threaten to move to the US, unfortunatly many of them are not US citizens and even if they are married to a US citizen visas for spouses of US citizens have been more difficult to get since 9/11 (Its been a point of contention nothing to do with Charedim in particular, just with Israelis wanting to come to the US)
And then there is the cost, It costs at $1000 a plane ticket per person to come to the US, a minimum of $10,000 per charedi family just for the plane tickets (assuming 8 kids and 2 adults) then you have to pay for US passports and then you will have to pay the costs of settling in the US (IE deposts on rent, purchase of all new possesions) at least anout $500 per person. which is $1500 a person MINIMUM (A US passport costs $110 if you dont expedite it) with 600,000 Charedim in israel it would cost $900,000,000 to bring them all to the US (and thats a low number, restttlement costs would likely be higher as shipping costs of things like Seforim would be expensive)
Finally there is not enough capacity on the airlines to transfer that many people (You just cant add flights due to landing restrictions in major US airports due to overcrowding)
So they are for the most part stuck there.
Frankly I think its a game of chicken, with both sides hoping the other backs down (usually the Pro-Draft people back down) but with both sides egging on the situtaion it seems headed for a showdown.
I do think if they cut back on benefits at least some will give in and go. Some will never give in and will need to go to jail for a few years before more will give in and some will never give in compating themselves to the Maharam M-rotenberg