I usually do it myself at the table. I can understand a restaurant's reluctance to do it-- cutting it in half in the back, can mess with the "presentation" of the dish at the table. In many cases (depending on what you ordered) they don't fix a plate and then cut it in half and serve you that plate where it's obvious half of it has been cut off, scraped off, or scooped off. For example, cutting a steak in half is going to have meat juices running all over the place. There is no way to remove half of a grilled chicken salad, or half of a pasta dish with melted cheese on top, without making a mess in the bowl-- better to prepare a dish for the table, and another one for the box.
My personal thought on it is, I let them present the dish to me at the table, the way they have designed the dish to be. Then I slaughter it myself.