unhedged
One possible reason for not allowing internationals in ED is that there might be more external factors that would prevent an international from being able to fully commit (the current loan climate, visas, etc.) No proof of this, just brainstorming.
Yes, Columbia accepts internationals for ED, so did it in the 2008 application season. And your brainstorming proved correct - internationals had commited to Columbia accepting ED terms and paying $6.000, but now they are left without non-co signed loans due to current financial situation.
These guys are now without admit from other schools (apps withdrawn or not even submitted), without loans and without deposits. Columbia commited to refund only $4.000 in the case they failed to secure financial package.
Very bad for Colubia's reputation, they seem to be the only M7 school that failed to secure non co-signed loans for internationals. Prospective applicants should be aware of this fact and to be very cautious when applying to b-schools next fall.