Transpiling Spread Operator to `Object.assign` is still ES6

Transpiling code with the Spread Operator like:

let obj = {
  one: 'value',
  ...({
    spreading: 'more',
    important: 'values'
  })
}

transpiles into:

var obj = Object.assign({}, {one: 'value'},
  ({
    spreading: 'more',
    important: 'values'
  }))

using Object.assign. And using the objectAssign option or adding a polyfill make this work in old browsers.

However, without manually taking care of this, technically buble produces ES6 output. This contradicts my expectations:

• Buble produces ES5 output
• Buble doesn't bring polyfills
• Buble throws an error if the code can't be transpiled into ES5

Are my expectations wrong?