There are numerous rules that go along with flying so-called ‘free’ (there are often fees for international travel, and for some there can be tax consequences as well).
One of the stranger ones on American Airlines is that employees cannot book conflicting paid travel and non-rev travel. In fact, they aren’t even allowed to list for non-rev travel on American is they have paid or mileage award tickets with the same origin and destination within 24 hours on another airline like Delta. This actually makes American Airlines less reliable for customers.