When I was growing up, I never knew anyone to take their own birthday cake to cut and eat at a restaurant. So when I first saw people do this, I was really surprised. My group of friends all do this and it makes me really uncomfortable. I feel so rude and cringe whenever they do it. “No we don’t want to order a dessert to give you money. We want to use your plates, knife and table instead for free.”
YABU - I do this
YANBU - It is rude