If there are laws of Waterdeep, I suggest they be posted elsewhere that all races, aligns, and faiths can visit.
Don't punish a Maskite that could not possibly even know the laws. They shouldn't be expected to follow them even on their best behavior if they didn't know they exist. Much less, would that hold any water in the courts I'd think.
From the Waterdeep sourcebook - Waterdeep has such law abiding citizens that the vast majority of laws are entirely unwritten and just fall under "reasonable discretion". Waterdeep is so incredibly hardlined on what is common sense justice, that they refuse to allow lawyers to practice in their courts.
It's an old system of law - nothing like our modern country's. If a guard witnesses something he feels he should act on, he does. He doesn't have a captain breathing down his neck about protocols, he'll always be backed up within good reason, and for the most part no real evidence is needed, and there is no bail.
Common courts deal with the lower crimes like leaving a 2,000 pound animal unattended in the city (this falls under common sense laws that are unwritten). The Three Magisters who preside don't even need to listen to the witnesses, they can just give judgment immediately. Everything that is serious, goes to the Lord's Court - that is tabled by the Open Lord himself and two others, magisters or lords. All crimes are classified under four "plaints" - think of them as degrees of crimes. 1st degree is any crime against the lords, 2nd degree is any crime against the city, 3rd degree is any crime against the gods, 4th degree is any crime against waterdeep citizens. Littering is a 3rd degree crime, for example. Leaving a horse unattended could be considered 2nd or 4th - depending on the complaint and how the Magisters see it.
I know it's difficult, but we have to separate ourselves from our modern conceptions of what the world is - and try to play within one that is relatively based off of 17th century ideals of law.
Listen up! People pay good money to see this movie! When they go out to a theater they want cold sodas, hot popcorn, and no monsters in the projection booth! Do I have to come up there myself? Do you think the Gremsters can stand up to the Hulkster?