I think you can resize auto columns, just not auto+fill ones (or collapse ones).
Making it possible to resize auto+fill columns would be nice and is on our list.
In the meantime, a good workaround is to add an item to the column right-click menu which turns off autosize (and resets things to normal on the second use):
You can use the script here to toggle columns between autosize and normal.
It's originally from the video about column resizing modes, at 18m31s.