The reason is that the primary liquid for which they are looking is Hydrogen Peroxide in high concentration, generally above 20%. The freezing point is 7 Deg F of 20% and even lower for the higher concentrations. It has to be in higher concentration to be dangerous. It is extremely difficult to maintain frozen hydrogen peroxide through the CP and higher concentrations are also highly unstable. The assumption is, and it is a good one, if it is frozen, it can not possibly be high concentration hydrogen peroxide.

That said, one takes a great chance that every screener knows the "frozen" protocol and follows it.