Forget the pictures. That can only raise a scandal that is irrelevant to the interesting subject you have raised.
Rank Aikido happens. I remember a period I used to complain to many students fell due to my BB, not to my actions. Despite the well known Israeli Chutzpa, the general lack of respect to rank in our society and the urging of Sensei and Sempai.
I believe you pointed correctly at the reasons - fear of pain and expectation of a technique requiring premature escape.
The only solution I know of is to be aware of this and careful. To insist on Uke resisting at times, and to look for better Uke as one's qualifier.