He must be playing computer games at home,isn't he,为何用isn't he?

别因为看到must而用了mustn't(禁止）
因为这句的主干是“He is playing ...."
而must只是表示肯定的判断，不影响主句意思。
所以用isn't he.

例：
You must know the truth,don't you?

如果must用于表示猜测的句子，去掉它，后面的动词根据主语的变化而变化，变成一般疑问句就可以了
He must be playing computer games at home变为
he is playing computer games at home,isn't he?

因为前面是BE动词啊，前肯后否，前否后肯。