OnlyLovers - I suspect you are just on this thread for sport - fair enough, if so, but I will answer your questions anyway.
I have worked in a supermarket many years ago and I can confirm that the management there certainly disliked people eating the food before paying. I was raised on occasion in connection with new prob he's to deal with shoplifting, as it made pursuing shoplifters more tricky. However, the large chain were not willing to introduce a policy where they put up signage asking for no eating. This was many years ago, but at the time, the issue was less prevalent and they preferred to turn a blind eye, although again I can confirm they wished people didn't do it - is that clear enough for you?
I find your question about evidence that shops would prefer people not to eat the products before paying for them bizarre - are you really suggesting any shops will encourage this or be neutral about it? At best they tolerate it, but this is absolutely not the same thing as being happy about it.
To me it seems to be a question of respecting the environment you are in. I wouldn't take food I am eating into a clothes shop and continue to eat it (even if there was no sign requesting me not to) because respect for the shop and their products, tells me that it would be inappropriate - I could make a mess, the clothes could be marked etc. if I went to a canteen style restaurant which required me to pay at the till before sitting down and eating, I wouldn't wolf my meal whilst in the queue.
It absolutely seems a lack of self-control and selfishness to me, when customers know that checkouts are intended to be gone through before consuming the products. To me, for adults to eat chicken or doughnuts or whatever is them saying 'up yours, I will do what I like' to the shop. And there are people who deliberately want to show he world that they can do whatever they like, wherever they like. I think it is right to call this selfish and lacking self-control.