It's just to cut off more additional work. Sure, it would have been no problem to _not_ kick out the player, but then Bethesda would have the need to add more stuff to the Vault like new dialogue lines, etc. It is really less work if you just close the door and say thats it, don't come back.
And you feel this all the time. You get something and think "cool, now this or this would be even more cool" but then nothing happens. It is one thing to make a "RPG" and another to make a good RPG with lots of outcome. But hey, then they would have needed to work harder with a much shorter timeframe and yeh. Then we will get a buggy game, etc. etc. etc. Reminds me a bit of Troika games. If they wouldn't have includet all the cool stuff and multiple ways in their games, they could have released less buggy games too.