I'm also staggered that there is no option to do this (at least not on my Mac version) as I'd always prefer to work on my PC and then to move this over to Ancestry. The option simply to upload (not sync) your tree works - but every time you make a change you would have to upload the whole tree (and images?) again. Or am I wrong here.
Surely 1-way sync should be an option for future versions.