I guess, post bursting implies not only a short period of time between posts, but also a short length of the post itself.
The length of a post does not determine the quality it adds to the discussion.
Many users wrongly relate post length to quality and this causes then to throw in a lot of fillers to make the reply look muh longer than it's necessary.
Managers add that character limit and set it very low as a baseline precaution, so users don't just write 'good observation there' and expect it to be counted.