Write your last name upside down?

Why is it an insult to the Chinese if you write their last names upside down? Is it the same as writing them backward in English? It's so unfair, some people have advantages than others!

看台上，两个素不相识的球迷争了起来。
“甲队准赢。我说错了，就把我的姓倒写！”
“甲队准输。否则，把我的姓倒写！”
“你贵姓？”
“姓田。你呢？”
“姓王。”

On the stadium, two complete strangers who were both soccer fans got into an argument.
"Team Jia will win, if I'm wrong, I'll write my name backwards!"
"Team Jia will lose, or else, I'll write my last name backwards!"
"What's your last name?"
"Tian, and yours?"
"Wang."
"..."