A very unfortunate situation! I think it's probably a two-fold cause:

1. A hotel is ultimately responsible for who they allow to check in, not Marriott. A guest could have an arrangement with the hotel that contradicts the Marriott's rules by the book because of a pre-existing relationship. Nowadays, Marriott is much more of a facilitator than owner of your booking / stay. Marriott will help you book, but it's the hotel that determines whether you can stay.
2. A Marriott customer support agent probably doesn't know much about how the rules used to be and for simplicities sake they probably make assumptions that you understand what you're doing if you make a specific request, I suspect if you had asked "how do I place a booking for my family on points if I can't be there to check in" it would have been a different conversation.

Marriott don't really care too much about how a booking is made, it's the hotel that is ultimately responsible for your conduct on the property and for getting paid: if you make a fraudulent reservation using a stolen account then it is the hotel who will be on the hook for any costs. Due to the rise in fraud (especially from stolen accounts) hotels are becoming more and more strict, nowadays it's all but impossible to check in without being the named guest on a points booking (and even on a cash booking in some cases).