I'm not really one to defend the TSA but unless/until the sippy cup gets into the secure area, there is no job for the TSA to do. Whether it was through the WTMD or x-ray doesn't matter. What matters is that the current rules require them to address the situation once it is brought into the secure area, not before. They can alert you all they want beforehand, but until it is brought in, nothing is going to happen. So, yes, I would say that they were doing their job, no matter how much you or I disagree with the job they were doing.