I don't think this is relevent. However you spin this it is purely the restaurants mistake and nobody elses. They should honour advance bookings - but they should do this by having a robust enough process that the wrong table isn't given to people without reservations.
Once you've walked in and been offered and accepted a table, thats that. If it's already reserved the restuarant has made a mistake and must therefore take steps to rectify that and that doesn't mean simply pushing the mistake onto the innocent people who walked in, asked for a table, and were offered one.
If the process fails, people affected by this process failure should be compensated as is the case when any business experiences process failure that affects customers.
Personally I'd probably move - I don't tend to buy into the whole 'But my night will be ruined!' crap, but that would be entirely my choice and not something I should feel obliged to do.
It's the easiest thing in the world to sort, too.
'I'm terribly sorry Sir, we accidently allocated you to the wrong table. If I could ask you to follow me to a different table, where I'll then fetch you the drink of your choice from the bar by way of apology'.
Job done, sorted, probably with a cost to the business of no more than £10 but crucially without affecting the customer loyalty of either party.