that makes sense with the rental stickers on bikes. Now that you mention it even though I am clearly not Thai, I have been waved through checkpoints in the moat area if I had long sleeves on and a helmet because I own my bike. So maybe it is not the skin so much as an easily identified sticker on the front. This isn't better though, that just kind of means that the police are intentionally targeting tourists and the motorbike shops are forced to assist them in doing this.