Using Geo IP wouldn't have been foolproof as you've pointed out, but it would have at the very least shown an intent to comply with the laws in the US.
The poker sites did not use Geo IP because they did not want to lose the revenue stream from the US, not because they thought it was technically insufficient... and thus the problem (or part of it).
The poker sites did not use Geo IP because they did not want to lose the revenue stream from the US, not because they thought it was technically insufficient... and thus the problem (or part of it).