It's laughable, really, because no other type of consumer article is as carefully controlled in Germany as tap water - it's even safer than bottled water, because tap water is continuously monitored by municple authorities.
So why don't Germans drink it? Because they were brought up not to like it! They were taught that soda water, which has more minerals and salts in it than tap water is "healthier" than tap water, and many cannot get the idea out of their head, that without carbonation, water tastes "icky".
And why can't you get tap water in restaurants? You can, but you have to demand it. Tap water in Germany is relatively expensive, and restaurant owners want to sell you food and drink. They don't really like the idea that you would drink something for free. So when you order in a restaurant, and you want tap water, order a drink from the menu FIRST, then just ask them nicely, if they will bring you a glass of tap water. If they look at you funny, just tell them you doctor told you, that it has to be tap water, then nobody will refuse you."