[tvon]

It's not arbitrary, it's just not explained.

[randallsquared]

If there are rules, the only plausible reason not to announce the rules is so that you can change them without having to give notice. This assumes that the rules haven't been announced, of course; maybe they have.

[tvon]

The guidelines are linked in the footer:

http://ycombinator.com/newsguidelines.html