Wow, $50/person seems like a LOT. When I worked in Hyatt reservations, it was typically $10/$20 more for each person over two.

Is this only happening on award reservations, or is this a regular hotel policy (at the hotels that charge it)?

It really is absolutely outrageous, but not a lot to do about it except try to find another place to stay. And complain to Marriott. And shame them on Tripadvisor.