It depends on what the cause of the diarrhea was. Unfortunately I've known of vets who would give a ton of treatments to milk you for your money (they don't all do that, but there are a few).

If it was something parasitic, then no, it's not that crazy. But if it was just that he ate something out of the ordinary or whatnot, then yes that it a bit crazy, considering usually just adding a bit of rice to the food will bind them and get them back to normal more often than not.