If all the seats on the airplane were paid for I agree. Some people do not pay for seats, and they save seats specifically for families, and what every other reasons.
At the time I booked there were no seats available for purchase next to each other it was booked about 2 days before I left. I reserved seats for the way home, as they were available. I feel I am entiled for the option to sit next to my family. If they didn't have room, they should not have sold me the tickets up front. If they charged me 20$ or 40$ at the gate I would have paid it. I would have accepted across the row, or any reasonable option. Not having the option would have drove me crazy, good thing stopping at the MK 4 -5 hours later would have fixed the bad mood!