That bip110 is not intended to be a solution to "spam", but it is intended to be a signal that spam/non monetary use is generally unwelcome.
There is a primary use case and not everything is that primary use case. This is why I quoted the word spam, because English is hard and using words without defining them leads to double speak.
Bip110 intends to signal that the most recent changes that welcomes experimentation using the op_return is not an invitation to redefine bitcoin's primary use case -- considering that it has limited resources, and there are not many (any?) use cases of a larger use of this field that don't intend to use up more of those resources for private secondary reasons.
Yes you could always use OP_RETURN unbounded technically, but it was awkward, now it is as good as a supported feature.
A fence does not stop people from entering your garden, but it does deter it.