A restaurant "is a business which prepares and serves food and drinks to customers in exchange for money."

McDonalds prepares and serves food and drinks to customers in exchange for money. This clearly makes it a restaurant. It may not be a fine-dining type of establishment, but it is a restaurant.

More pretentious is all the hipsters that look down on restaurants like McDonalds simply because it doesn't fit their narrow, preconceived notions of what a restaurant should be.

Agreed. Long back my office used to have 'canteen' for food. But nowadays it is considered downtrodden and 'Cafe' is used in place.

At least they're not yet so pretentious as to call it an "eatery" which seem to be the latest trend among middling restaurants trying to distinguish themselves.