Why would you not be able to review the diff with a local tree? If anything that's much easier in a local environment.

The GUIs by nature hide detail: the pull request becomes a thing to be "displayed" instead of "explored". It's a problem that requires careful attention to detail. I don't know that I actually disagree with Atlassian's decision, but the fact that it had to be made isn't evidence that it's the "right" solution either.