That was never done because its a tricky one - what do you randomize? Whether its flipped or not? I can think of uses for all the different combinations of this, and the problem is when we discussed it, we couldn't find a nice way to present it. The current behaviour seemed the least problematic of the other options.
Its really easy to patch in though. Look for these lines in tk2dTileMapSceneGUI.cs and add the lines in bold -
if (tk2dTileMapToolbar.mainMode == tk2dTileMapToolbar.MainMode.BrushRandom) {
int gridWidth = 1 + rectX2 - rectX1;
int gridHeight = 1 + rectY2 - rectY1;
workBrush.tiles = new tk2dSparseTile[gridWidth * gridHeight];
var rng = new System.Random(randomSeed + cursorY * tileMap.width + cursorX);
int idx = 0;
for (int y = 0; y < gridHeight; ++y) {
int thisRowXOffset = ((y & 1) == 1) ? xoffset : 0;
for (int x = 0; x < gridWidth; ++x) {
int spriteId = srcTiles[rng.Next(srcTiles.Length)].spriteId;
if (rot90)
tk2dRuntime.TileMap.BuilderUtil.SetRawTileFlag(ref spriteId, tk2dTileFlags.Rot90, true);
if (flipH)
tk2dRuntime.TileMap.BuilderUtil.InvertRawTileFlag(ref spriteId, tk2dTileFlags.FlipX);
if (flipV)
tk2dRuntime.TileMap.BuilderUtil.InvertRawTileFlag(ref spriteId, tk2dTileFlags.FlipY);
workBrush.tiles[idx++] = new tk2dSparseTile(
rectX1 + x + thisRowXOffset, rectY1 + y, editorData.layer, spriteId);
}
}
}