Then they shouldn't ask for beta-readers or editors.
They don't have to agree, but they shouldn't be confrontational.
Don't confuse confrontational with confusion. Often, when someone thinks they're doing the right thing, they'll try to justify their usage. They'll only agree to change when they finally learn why what they're doing isn't standard English grammar.
By the way, my editors continually go back and forth about commas vs. semicolons, which is generally a round-about way of dealing with comma splices. Generally, the best way to deal with them is to break the sentence up into two distinct sentences, but if there are conditional clauses involved, it's often difficult to break them up cleanly. That's why I'll state my position, along with my current understanding of the underlying issue, and present an alternative, expecting further pushback from the editor if I'm incorrect. But I expect pushback, sometimes forceful pushback. I might have to be dragged, kicking and screaming to the stream, but once they, I'll drink my fill!