Two possible answers:

1. The TSO wasn't unsure about the situation in question. He was sure. He was completely wrong, but he was sure. Consequently, from his perspective, there was no need to call a supervisor.

2. In most areas of life, people in authority hesitate to admit ignorance; it's seen by many as a sign of weakness. Consequently, we cover up our ignorance by reasserting our position, even if it doesn't hold up. (This isn't solely the purview of TSA, of course ... there's an old lawyer's saying: "When the facts are against you, argue the law. When the law is against you, argue the facts. When both the facts and law are against you, argue louder.")