50 gallons isn't much water per person. Remember, flushing the toilet a few times a day per person, a shower per day, washing dishes, washing clothes. Even little things like leaving the water running while brushing teeth adds up to a few gallons. And you'd better hope that you don't have a slowly leaking toilet. 50 gallons per day is well below average in this country.
Anyhow, until there's a reputable site...
50 gallons is too low IMO. I'd rather it be alon lines of 75gallon limit per day, with larger fines above it. Most of the water waste is not coming from typical households,.. it's coming from large homes with large yards and it's coming from golf courses and other businesses that use large amounts of water.
The 50gallon limit would hit poor/middle class in order to serve the well to do and privileged who could use well above that amount and happily pay a $500 fine.
Don't hit the poor/middle class for water waste that is not equally distributed amongst the populace. $500 fine for the well to do, even if once per month, they don't give a shit, $500 fine for the rest of us, particularly if monthly, is a big deal.
But yes, none of this is happening.