"Taking up a seat without ordering" is frowned upon. It's okay to dislike that as a restaurant owner. It's universally disliked, and barring exceptional circumstances, it's not a cool thing to do.
That is what the problematic behaviour with your described case is. "Free tap water", however, absolutely should be the standard. In Belgium, you will generally be refused tap water even if you're a paying customer with a 100 EUR bill. Whereas in France, it's illegal to refuse serving tap water… think about that.
If you're not allowed to drink tap water, you'll be drinking bottled water which creates plastic/glass waste and promotes a business that essentially repackages tap water.