Passing over the merits and free speech issues, why was this Russian case tried in a military court?

Easy. It's Russia, again. A freelance journalist is convicted by a military court on cockamie charges of "justifying terrorism." The New York Times has the details here. At least she wasn't jailed. Excerpt:

The case against Ms. [Svetlana] Prokopyeva revolved around a brief commentary she wrote in 2018 after a Russian teenager, a self-declared anarchist, blew himself up inside a branch of the secret police in Arkhangelsk, near the Arctic Circle.

She held the government responsible for the attack, which killed only the 17-year-old bomber, arguing that nonviolent means of protest like street protests had been steadily closed off by often violent security officers.

“Cruelty breeds cruelty,” the journalist said in her commentary, which was broadcast on the Pskov affiliate of Ekho Moskvy, a Moscow radio station, and also published on the internet.