Well, if the butter is frozen, you could argue itīs a solid, not a liquid.

When they start arguing "but it could melt" you could respond "so does almost anything under the right circumstances"

You could also question why glasses arenīt confiscated even though itīs considered to be a liquid, not solid. (at least in the field of thermodynamics and other fields of chemistry)

At that point the agent will probably/hopefully give in.