Looking at the test case/expected error, you are correct and should be cloning the object.
Warning: `div` was passed a style object that has previously been
mutated. Mutating `style` is deprecated. Consider cloning it
beforehand. Check the `render` of `App`. Previous style:
{border: "1px solid black"}. Mutated style:
{border: "1px solid black", position: "absolute"}.
https://github.com/facebook/react/blob/fc96f31fade8043856eb7bc34c56598c4a576110/src/renderers/dom/shared/tests/ReactDOMComponent-test.js#L128
I would guess it's similar to the reasoning for props - it saves you from passing a mutable style object around everywhere and ending up losing a lot of the ease of reasoning that React is really good at helping you with.
The props object is now frozen, so mutating props after creating a component element is no longer supported. In most cases, React.cloneElement should be used instead. This change makes your components easier to reason about and enables the compiler optimizations mentioned above.