Nobody collects I-94 when you drive back to Canada from USA. So 移民局的記錄始終是過期的TD if no renewal made.
B1 entries from Canada may not be logged to CIS database, s/he would have no new record (no visa stamp) when s/he returns to the US later.
There is a hole (no out of status problem) for Canadians somehow. CIS does not care about this.