Let's make the math a little more favorable. If the TSA caught 99% of liquids people try to bring through and then randomly tested 5% of them, the terrorist faces a 1% chance of success in each attempt, and a 4.95% chance of arrest. Assuming they repeat upon the most likely (liquids confiscate and not tested) outcome, they face an overall 16.81% probability of eventually getting the liquids through, and a 83.19% probability of arrest. Now, a 16.81% failure rate ain't great (and my assumptions are probably favorable), but that's why we need a layered security system.

On the other hand, if the TSA tests 0% of the liquids, the probability of eventual success jumps to 100%...

To test 5% of confiscated liquids, the TSA would have to detain tens of thousands of passengers.

Good thing for me I wasn't arguing that the TSA was actually testing the liquids and thus not wasting everyone's time!

Edit: on the other hand, I'm not sure it's so infeasible... If 5% of passengers have liquids, and 5% of those get checked, that still seems like a smaller fraction of people than those that get extra security already. Simply having agents open the bottles and smell them (this actually happened to me in china!) would be somewhat effective...