根据英语习惯，某些成双成对的名词短语通常可以省略冠词，网友的句子就属于这种情况。其实加上冠词也是可以的：

Do all westerners eat with knife and fork?

Do all westerners eat a with a knife and fork?

比较而言，不用冠词的表达语义更抽象。又如：

They eat with (a) knife and fork. 他们用刀叉吃东西。

The child was awkward with (a) knife and fork. 这孩子使用刀叉还不利索。

Do you eat rice with (a) knife and fork or with chopsticks? 你用刀叉吃饭，还是用筷子吃饭？