来自 点击事件文档：

From the click event documentation:

默认情况下，Knockout会阻止点击事件采取任何默认操作。

...

但是，如果你想让默认的点击操作继续，只需从你的点击处理程序返回true 函数。

By default, Knockout will prevent the click event from taking any default action.
...
However, if you do want to let the default click action proceed, just return truefrom your click handler function.

因为你的点击处理程序而你的单选按钮被重置了修复它你只需最后返回true ：

So your radio button is reset because of your click handler and to fix it you just need to return true at the end:

click: function(){
   alert('Hi');
   return true;
}

演示 JSFiddle 。