Precisely. If the DM established that this was indeed how cleave worked, then why wouldn’t all the fighters of that world be aware of it and take advantage? Now whether you want the ability to work like that is another question.
That would arguably be the consequence, with the caveat that this type of thing sets a tone of parody (or at least extreme-beer-and-pretzels) whether that's what you were aiming for or not. If the group is fine with this or simply doesn't care, it isn't an issue.
As I see it, the problem arises when people do care, but a rules lawyer finds a "loophole" in a weakly written rule or a combination of rules that weren't written in consideration of each other. Then that player insists on their exploit being the correct interpretation, to the point of outright arguing when the DM steps in to restore sanity by closing the loophole.
If you've read the whole thread I can point to at least two examples. One where the hypothetical DM was accused of outright cheating (multiple times no less) for not allowing the hex chicken, and another where (to paraphrase) the "DM" was accused of making an arbitrary ruling to prevent the player from taking perfectly legitimate actions.
Now, this being an internet forum, it's entirely plausible that both of those folks would be more considerate and accepting of the ruling when face to face with their own DM (as opposed to a relatively anonymous stranger such as myself). However, I've certainly seen such behavior in person at tables I've sat at. I seriously doubt I'm the only one who ever has.
That's the crux of the problem. If the DM allows it, he's permitting the destruction of the immersion of the players who do actually find that sort of thing important. The world loses some verisimilitude because the player doing it is not engaging with the world as a living place but rather as a game whose rules are made to be hacked for any advantage the player can find. IME, it's not unusual for players who enjoy immersion to take matters into their own hands and end up in an argument with the player doing this.
If the DM tries to prevent it by saying no, an argument with the loophole player is likely to ensue, because they won't accept the DMs ruling.
I mean, if you as a player come to me and propose it, and I say no, and that's the end of it, no problem. IME, it rarely works out that way.
