The "I'll have the beef" way of stating an order doesn't come off as rude or imposing. There are only a few ways of saying what you want and people like variety, perhaps it's that which makes it a normal phrase

If you don't like being wrong ("I'll have the spaghetti"; "no you won't, we've sold out") you can use phrases like "may I get ..." or "I would like ..." or in reply to whatever the waiter asks "the pasta marinara" with no introductory words