IANAL either, so my own legal theories are as creative as yours, but I'd like to offer the following data point: All unrestricted open-source licenses that were written by actual lawyers, from MIT to CC0, have found it necessary to include such a liability clause.

In what sense is the MIT license "unrestricted"?

In the sense that when people want to use a piece of MIT-licensed software in another piece of software, they don't in practice find themselves restricted from doing so by the conditions of the license. "Permissive" might be a word I should rather have used.

The MIT license does place one specific license restriction on its users. Specifically: "subject to the following conditions: the above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software"

This is what I was getting at. The MIT license has restrictions, so calling it "unrestricted" doesn't make sense.