Because she actually didn't do much. In fact, she almost got herself killed because of it. The spell she used only disables magic spells and communication spells and by extension, spatial magic. She only delayed Rimuru by a few minutes.Considering what she did and how much damage was caused to Tempest it always bugs how it handles her punishment. Without spoilers for the main story I know the reason Rimuru gives but doesn’t seem right after seeing how much sadness was caused by her.