This fall, Sierra Riddle queued up at security at Los Angeles International Airport with a tincture bottle of THC oil — the psychoactive ingredient in cannabis — in her purse.

Ms. Riddle, 31, a nursing assistant from southern Oregon, was traveling with her son Landon, 9, and uses medical marijuana to treat his severe nerve pain from chemotherapy, as well as her own chronic pain. She was on her way to a medical conference in Dallas to talk about her son’s medical marijuana use and was “praying and meditating that we’d make it through security,” when a Transportation Security Administration agent pulled the bottle out of her bag.

“It’s just botanical oils,” Ms. Riddle said she told the officer. “But this was L.A. — they’re hip to the game and so they knew what is was.”

To Ms. Riddle’s surprise, the officer told her that, while it’s illegal to fly with marijuana and he was obliged to call the police, instead, he would just throw the bottle in the trash and wouldn’t report her.