O Outlook não suporta alertas / lembretes de calendários públicos.
Do MS KB 186689 "XCLN: Lembretes não são iniciados para itens em pastas públicas":
Because public folders are not specific to any one user or profile, there is no one for the reminder to remind. To receive reminders to specific appointments or meetings, use your mailbox calendar to make appointments and schedule meetings.
Você pode obter a funcionalidade usando um complemento como " Função de lembrete estendido para Microsoft® Outlook®, Public ShareFolder e Microsoft® Exchange Server ":
Public Reminder Add-In provides reminders from any email, calendar, tasks or contacts folder in Microsoft Outlook, Public ShareFolder or Microsoft Exchange Server.
This allows you to set reminders on items in any folder in your mailbox or PST file, secondary PST files, archive PST files or Public Folders of Public ShareFolder or Microsoft Exchange Server.