Respecting type class laws isn't optional: The laws are an intrinsic part of the abstraction's definition, so if your instances don't respect them, your code is simply wrong.
In any case, you might argue it isn't a mathematical monad, but it can still be a Haskell monad as long as it implements the typeclass.
I agree it makes the most sense if it implements the laws: Otherwise, it's like calling something a Stack<String> when it really deletes your home directory whenever you peek an element. Technically anything that implements the Stack<> interface is a Stack in the language, but it's not an abstract Stack.
This attitude is unacceptable, because it undermines the meaning of abstractions, on which much of the Haskell ecosystem critically depends. It is also a potential turnoff for those who want to learn Haskell but find the learning curve too steep: "What use is investing so much time and effort learning abstractions that people will routinely break anyway?"