This comes from a fundamental difference of opinion about pricing and the role of water. In most countries, water is considered an essential accompaniment to food (in several, it is illegal to refuse it to guests in eateries); in Germany, it is considered a way to recoup costs on alluring prices for meals with which restaurant owners would otherwise be cutting their own throats. Especially when dining on an attractively priced midday offer, customers who do not order a beverage of some sort are probably costing more than they’re paying. Elsewhere, restaurants would rectify this by upping the price of their food; in Germany, the solution is to get arsey if the customer doesn’t play the game and order some form of high-margin liquid refreshment.