Why doesn't USCIS tell you about INA 222(g)?

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My mother-in-law came here in 2007 for a six month stay. We filed for an extension in May 2008, a month and a half before the expiration of her I-94. Her I-94 expired and she subsequently got an RFE and then was denied in late October of last year. She left within a week of receiving the denial letter. Before she left though, we made an InfoPass with the USCIS Denver office and they told her the denial would have no effect on her 10 year visa. I called the local CBP office and was told that as long as she left ASAP, she had nothing to worry about.

Having read some stuff online that made me question what CBP and USCIS had told me, I emailed the US Embassy in Manila last week to inquire about the status of her 10 year visa and this is what they told me.

In general, one who has overstayed the authorized period of admission may no longer use the visa with which he or she entered the United States under U.S. Immigration and Nationality Act Section 222(g). This section renders void the visas of nonimmigrants who remain in the United States "beyond the period of stay authorized by the Secretary of Homeland Security." In this situation, one needs to reapply for a new nonimmigrant visa in his/her country of nationality. One’s eligibility for a new visa is determined at the time of interview by a consular officer. Rest assured that all applications are given full consideration consistent with United States law and visa regulations.

If that's the case, then why didn't the denial notice from USCIS say anything about 222(g) and why didn't CBP know anything about it either? I could see someone just going back home after being denied and then later trying to use their visa again, completely unaware that it was automatically cancelled.

If we had known that this would be the consequence of a denied I-539, we would have had my mother-in-law leave for home as soon as it was clear she would not receive a decision prior to the original expiration of her I-94. I don't know why these agencies can't all get on the same page especially on an issue like this where they all have a part to play in it.