Well, really the booked sleeper passenger has paid for the whole room, so it IS paid for. Amtrak has still gotten the same revenue. In terms of who pays for what, someone with a coach ticket sharing the room with a sleeper passenger is indistinguishable from an open sleeper ticket (actually, Amtrak may get MORE revenue, since the open sleeper ticket will have been a the lowest value bucket, where the coach ticket may have been at a higher bucket). So it is in no way unfair to other passengers in terms of what they were charged.
From a security viewpoint, it is problematic, and personally I would not feel comfortable sharing my room overnight with someone I have just met. Passing the time of day with a new friend in the sleeper for awhile is different and in some ways it is not different than inviting someone to your hotel room. However, unlike a hotel, the other doors are not lockable except from the inside and there is luggage on an open rack, I think from a security standpoint a firm rule against passengers without a sleeper ticket is justified.
By the way, until the streamline era starting in the 1930s, private rooms in Pullmans were very much the exception and not the rule. Standard Pullmans were 14-1, IIRC, 14 sections, one compartment or drawing room. The berths WERE sold individually, so complete strangers could be and were in the upper and lower berths (still are on the Canadian). So in George's day, sharing common space in sleepers was the way things were "on this side of Pond".