Not opening present in front of the giver

Whenever anyone gives a present to me and he/she asks me to unwrap the present in front of him/her, I always refuse to do so.

That friend usually asks, "Why?"

"Because it's taboo in Japanese tradition to open a gift in front of the giver..."

Then they let out a big ZWT and say,

"But we're not in Japan, and we're not Japanese..."