I'm sorry, but that just doesn't seem right. If you've gone to the trouble to make your ADR's at 180 days, and managed to get into a popular, hard to reserve restaurant, and then a month before your trip, your traveling companions lose a job or something and can't go, they shouldn't be able to turn you away at the door.
If it's a popular restaurant, then they will probably be fully booked and not taking walk ins, so it would make better business sense to allow the party of two to dine at a four top than to lose the revenue from the party of two when you aren't going to allow a walk in party to replace them (yes, I realize they will probably want to put a party of four there, but I'm saying that the overall head count for the restaurant for the night will be two less than it would have been, in this example).
I run into a similar problem making ADR's though OP, that doesn't make any sense either. As a solo, I would be seated at a two top, but many times it won't allow me to make the reservation for a party of one. I have to book a party of two (same size table) and then adjust at the restaurant when I arrive. I always try a party of one first, to see if availability comes up, but I would say about 30% of the time, I end up having to book for two.