Diary of William Peter Dortch, 351st BG, 508th BS, Polebrook, UK

This may seem an odd question, but I am curious about the keeping of a diary by air crewmembers. Was this activity ever considered illegal or were there specific regulations prohibiting this activity or limiting where these diaries might be kept? e.g. not to be taken on missions to avoid these records from falling into enemy hands?
 
Specifically I have heard that this activity was forbidden in China prior to the withdrawal from these bases in late 1944. Is there any credibility to the prohibition on keeping a diary in that theater?
 
Any help on this subject is welcome.
 
Thanks,
Scott