Any restroom that is susceptible to use by the masses - public toilets on the street and in fast food restaurants - is a pay toilet in Prague, and they are relentlessly policed by attendants. That's just an observation, not a complaint. I don't blame them. In fact, I'm all for it. It costs a lot of money to keep a public toilet clean! 5 or 10 kuronas per use.
I do think it's in odd juxtaposition to their unspoken civil agreement re public transport. Trams, subways and buses are also not free - but they are! About 0.01% of the population actually buy a ticket. They don't even have turnstiles in the subway. It's all on the honor system, and obviously nobody is upset that only 0.01% of the people pay. I never once saw any official checking people's tickets. And I never once bought a ticket. For the first few days Brigham and I were unaware that a ticketing system exists. We finally saw someone actually use a ticket!
On the other hand, TRAINS are definitely NOT free. Which brings us to the train station, where a strange law prevails.
All other restrooms in Prague have one set fee, either 5kc or 10kc. But at the train station, you pay 5kc to pisoár, or 10kc to kabinky! The blue lighting in the restroom is not universal, but is very common. I'm not sure if it helps kill germs, or sets a relaxing mood, or what. When you walk out of such a restroom, the world has a bright yellow glow.
In review: If you use the restroom at McDonald's in Prague it will cost you 5 or 10kc. In the train station, it depends on what you are using it for.
A large packet of ketchup at McDonald's will cost you 20kc. No free ketchup. But you can buy a beer in McDonald's. But I did not. I'm not that fond of beer.