请教一个SAT和GMAT对一个解释比较困惑的SC问题：

清零

今天阅读了一个李开复的帖子： "Many people followed the news, and sent me their condolences about my recent predicament. ".  

如果是我我就可能会写为："Many people followed the news and sent me their condolences about my recent predicament. " (without the comma)

我发现在美国参加高考的孩子，就是坚持认为李开复的用法是对的。是不是GMAT的语法准则跟SAT有所差别？或者矛盾？
比如 Manhattan GMAT Sentence Correction 4th Edition: 如下原话：

Do not use a commna before 'and" to seperate two verbs that have the same subject. Either dliminate the comma or add a subject to the second verb,creating a second main clause.
Wrong: Earl walked to school, and later ate his lunch.
Right: Earl walked to school and later ate his lunch.
Right: Earl walked to schoo, and he later ate his lunch.

大家发言解释一下？多谢