You would cleave anyway, inviting someone to leech isn't affecting your time and effort spent.
On the contrary, I cleave on occasion because people willingly pay gil for it. The only exception is if I'm after key items from boxes, and at that point I am not inviting random people.
but most likely its only aimed at the cleavers who have another character autonomously shouting around the clock.
Yes, it is likely, but at the same time they do not specify "around the clock," they simply make it sound like if you have a mule shouting while you cleave to pick up people, that it's "disrupting" and you "may" be banned for it. As it has already been pointed out, Square Enix does not exactly have a great track record with mass bannings hitting only guilty people.
so why pay for something you can get for free?
I often ask myself this also. I do not have an answer for it short of this; "people do pay for it because they are lazy."