gorkai's page

Bottom line is that the GM made a ruling and the fact that it has been argued so much here proves that it is a viable possibility. Except when the ruling is patently unfair or going against the rule, the GM has a right to make a decision and stick with it.
I tend to allow the extended crit. range when I GM but have been in games where my druid had to use that spell with a standard 20/x2 Critical and had no problem with it.

Most scenarios where the players have to deal with cold contain a warning to that effect in the text. If you hinted at your players that they should buy cold gear and they did not do it, well then, I would have done the same thing and asked them to show me a proof of purchase.
We can discuss where they should write it but the fact remains that they could not back their claim.