There usually is no explicit treaty. That's why it's a hard problem. People become so used to those around them being mostly reasonable* most of the time that when white supremacists show up to promote genocide, no specific line has been crossed, so those around them just continue to act like this is just another thing to be 'tolerated'.

(*reasonable by mainstream standards, which already allows for a lot of unreasonable stuff, like billionares polluting the environment in the name of profit, which is somewhat related in that not bothering to define what is okay and what isn't contributes to the persistence of some really bad problems)