Firstly, do you think the mods of 4chan could really handle the population of /b/ being loosed on the other boards and wreaking havoc? I don't. It happened last summer a few times when we stole some of their GETs, and it was pandemonium in here for hours.
Secondly, there's no rule against shitposting, or half of /a/ would be permabanned already. Blatant rulebreakers, sure, but just bad posts? Grey area.