I'd call it stupid, but I can't think of a "less stupid" rule.
There are just more factors than distance that contribute to a gap being closeable or not. A quick, relatively painless crash for a strong climber, 2k from an uphill finish (like stage 1) might be closeable, while a really bad crash 15k from a flat finish might not be for the same guy, especially with a damaged bike and nobody to help pull him back up. This could even be the case on the same stage, if kilometers 15-2 were flat, and the last two uphill.
Here's the text:
Originally Posted by UCI Rule 2.6.027
In the case of a duly noted fall, puncture or mechanical incident in the last three kilometers of a
road race stage, the rider or riders involved shall be credited with the time of the rider or riders in
whose company they were riding at the moment of the accident. His or their placing shall be determined by the order in which he or they actually cross the finishing line.
If, as the result of a duly noted fall in the last three kilometers, a rider cannot cross the finishing line, he
shall be placed last in the stage and credited with the time of the rider or riders in whose company he
was riding at the time of the accident.
(text modified on 1.01.05)