Pretty sure the GP doesn't care about the law in the analogy; the design of the car itself impedes your ability to drive with your feet.
In the same way, slack is frictionless to post one-liners; you have to go out of your way to post a multi-liner (and I personally have the constant worry I'll miss the shift and post half-finished in such systems).
It doesn't matter if the law (company) says to drive by hands; you have to be really intent on driving with your feet to get into a car and do such a clearly unnatural thing. It's difficult to imagine someone new getting in and thinking this must be the correct position, because the car does not support it naturally. It can be done, but only while dealing with friction.
Then compare slack to a bbs forum, where people naturally post paragraphs. Not coincidentally, the act of posting has more friction; CLRF is a newline, posting is a button off at the bottom of the page. To get a better ratio of value (writing) to friction, you'll simply have to write more and post less.
The law (company) can enforce unnatural behavior onto its populace, but when the law is unclear or unstated (or ignored) on the particular matter, the natural activity depends on the environment.
And slack is very clearly predisposed to a particular kind of posting.