This is probably a bug and was maybe done in an earlier LabVIEW version where this particular sanity check was not implemented or where the functionality was slightly different.
There is no conceivable use for of writing a "double-click" to a property node. What should possibly happen?
Since the value automatically reverts to -1 after the propery has been read, the property write in the example does nothing useful. Please report it to NI so the example can be freshened up a bit for the next release.
Maybe you could tell us what you want to achieve and we'll find a solution. Writing to a double-click property won't be it. Good luck.