If you call in and book a table for a certain number of people, the restaurant or bar takes you at your word and stops other people sitting there and it - when the occasion demands - makes sure you have enough food for the night. So if you decide to cancel at the last minute, it costs the venue money.
Now I know that it's not easy to corral a full house together for a night out, but surely it doesn't take much time or effort to call the venue and cancel, and the earlier the better, especially as you could be costing a venue for food.
One of the big problems is that people think it's fine to simply cancel at the last moment. It isn't - the venue has probably organised its staffing and food around what is likely to be sold on any given evening and a late cancellation can throw all that into severe disrepair. And then people complain about the prices.
It's fair enough for bars to charge a bit extra to cover the cost of non-attendance, especially when they've been given a certain number of prospective customers.
So, if you're going out in an organised fashion, let the venue know, And if your mates can't be bothered, pick up the phone and let the venue know. It's only fair, after all.