Are you sure about that? Haven't they consistently captured and locked up metahumans in Star Labs? Isn't that being judge, jury and executioner? And it's not like Team Flash has told the authorities about the cure.
Yes they were acting as judge, jury and executioner going all the way back to Season 1 when they would just toss metas in the pipe line without due process, and that was a point of debate back then too whether or not Team Flash had any right to do that.