@SingingInTheRainstorm - this was not just because it was the PIL - the OP has already said that she would have felt the same whoever it was that turned up.
Where I used to live, I had a number of friends who used to drop in unannounced all the time, and I liked it. It worked for me, at the time - the boys were all over 3, so were out at playgroup or school, and I welcomed the company. But I can remember back to the time when they were much smaller, and they'd have a nap after lunch - and I loved that bit of time to myself, to watch Neighbours and Home and Away and/or nap in my chair in the front room - it was very precious, and helped me hang onto the remaining shreds of my sanity, and if people had popped in unannounced then, I would have felt deprived, in the same way the OP does. Luckily I hadn't made that group of friends, at that point, so there was very little unannounced popping.
I can see both sides of the step father who drove 170 miles and was turned away. On the one hand, as @NavyandWhite says, it seems pretty mean to turn away someone who has driven that far, without so much as a cup of tea - but on the other hand, as @mathanxiety says, it is pretty strange to drive that far without having contacted the person you intend to visit, to make sure they are in and in a fit state to welcome company.
Maybe the compromise would have been a quick cuppa, and then send him on his way - a bit less mean but would still have made him think about phoning ahead the next time?