I might be a performance issue. The decompression may have been determined to be too slow to meet their performance goals when for example flicking through a few photos. Perhaps just a marketing decision?
I do not really understand the precise issue, and please do not take the time to explain it to me:-), but I noticed the following and thought it might help.
https://itunes.apple.com/gb/app/idow...541063049?mt=8
iDownsize allows the compression of selected photos or entire albums.
iDownsize allows you to take along your entire photo repertoire - wherever you go.
Would be no good if iTunes simply re-synched the altered files though I don't suppose.