这是预期的行为。将动作栏设置为 Activity 它的作用是 Activity 决定要显示的菜单。
这是的文档和方法声明 setSupportActionBar 从 appcompat-1.0.0 附属国。您可以阅读所有内容，但我从javadoc中显式地选择了一个句子，并在下面的片段中提到了它。

/**
 * Set a {@link Toolbar} to act as the {@link ActionBar} for this delegate.
 *
 * <p>When set to a non-null value the {@link #getSupportActionBar()} ()} method will return
 * an {@link ActionBar} object that can be used to control the given toolbar as if it were
 * a traditional window decor action bar. The toolbar's menu will be populated with the
 * Activity's options menu and the navigation button will be wired through the standard
 * {@link android.R.id#home home} menu select action.</p>
 *
 * <p>In order to use a Toolbar within the Activity's window content the application
 * must not request the window feature
 * {@link AppCompatDelegate#FEATURE_SUPPORT_ACTION_BAR FEATURE_SUPPORT_ACTION_BAR}.</p>
 *
 * @param toolbar Toolbar to set as the Activity's action bar, or {@code null} to clear it
 */
public abstract void setSupportActionBar(@Nullable Toolbar toolbar);

重点是这句话：

* ... The toolbar's menu will be populated with the

 * Activity's options menu and the navigation button will be wired through the standard
 * {@link android.R.id#home home} menu select action ...

这意味着你必须重写 onCreateOptionsMenu 方法，以便在活动中为操作栏创建菜单：

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.resource_id, menu);
    return true;
}

默认情况下，如果不重写此方法，将填充菜单
... 带有标准系统菜单项。
引用的来源（注意，我删除了 onCreateOptionsMenu 与此问题无关的）：

/**
 * Initialize the contents of the Activity's standard options menu.  You
 * should place your menu items in to <var>menu</var>.
 *
 * <p>This is only called once, the first time the options menu is
 * displayed.
 *
 * <p>The default implementation populates the menu with standard system
 * menu items. 
 *
 * @param menu The options menu in which you place your items.
 *
 * @return You must return true for the menu to be displayed;
 *         if you return false it will not be shown.
 */
public boolean onCreateOptionsMenu(Menu menu) {
    if (mParent != null) {
        return mParent.onCreateOptionsMenu(menu);
    }
    return true;
}