Why was this case tried in a military court?

A military court in Gaza has tried three civilians for participating in a Zoom conference with Israelis. According to this account:

Hamas authorities convicted the three activists under military law, rather than the civilian penal code. The conviction — weakening revolutionary spirit — harkens back to statutes encoded in 1979 by the Palestine Liberation Organization, well before the establishment of Palestinian civilian legal institutions in Gaza and the West Bank. “Bringing civilians before the military courts is a clear violation of the Palestinian Basic Law,” the detainees’ defense said in a statement on Monday. Human Rights Watch director for Israel and Palestine Omar Shakir hailed the activists’ release in a tweet, but called the court’s decision to convict them “outrageous.”