rgs and myself might be poles apart politically, though I suspect not. One thing we do have in common is that we live with the people, the Thai people. You cannot explain that, at least I cannot. It becomes an instinctive response in any situation. rgs buying the doughnuts i totally get. A tip would be an insult.
Another example, where a tip was not a tip. Yesterday. A lottery seller on his bike. Boy, I often mention him as we are very close. Boy was buying and I in my perverse way made up a random winning number. Seller did not have it. I never buy lottery tickets, never gamble. That became a long conversation after the event. I asked the seller how much profit did he get from each ticket sold, he told me nine baht. I gave him ten baht and want no ticket. Call it making merit. All got it, it was not an insult, a friendly gesture as I never buy a ticket and the guy makes pea nuts.

PS i never give beggars anything as i know it is mostly organised crime. A guy on a bike , Tebbit comes to mind.