Basically, lava sets flammable materials on fire even if it's separated by a non-flammable block. I think it does need at least 1 open block nearby to start a flame. Even if it's meant to be like that...it'd be suggested to stop that anyway..modders who make anti-grief mods probably don't know that and the victims pay the price. I didn't know of it until now and it burned 7 floors of my house down.