Your pointing this fact out does not defend the policy; nor does it undermine the analogy. The comparison was drawn simply to show that there must be a disincentive attached to attempts to breach security. The analogy shows that there are two classes of item that both potentially represent threats (in this way, they are analogous), and yet only one of them is subject to penalty (in this way, they are differentiated).
The illegal act that I was referring to in my post (vis. attempting to illegally enter a restricted airside area with an unlawful quantity of liquids) referred to the conduct of entering an area with said liquids. Again, for the sake of brevity, I didn't write out that entire phrase in full each time, although it makes no practical difference when we are discussing security screening procedures at entry points.