They borrow - from family, or a private loan from the bank (with crappy interest rates). Or they dip into their savings.
In any case, there's a healthy minority of people who get accepted into ED and don't end up going.
Every school got spooked by the credit crisis -- in the US, Citi was the program lender (with schools acting as the cosigner) for MBA loans at all the top schools - until Citi pulled the plug when they became a penny stock. All the schools panicked, but still managed to come up with their own loan programs somehow.
Except Columbia.
You can take that as you will.
And now, they expect people to fork over $6K *and* if funding falls through for that student -- they have the *nerve* to keep your $6K?
It goes to show how ridiculous this Early Decision has become. They will defend it to death with spin, spin, spin, but it really is just shows the huge inferiority complex that Columbia has had with Wharton and HBS (the two schools they lose most students to -- and now probably also Booth as well) which is why this whole ED silliness is in place.
In any case, if funding is an issue for you -- don't apply to Columbia. It's that simple. Saves you the headache.