So if I want to post from a mobile device, I have to wait considerably longer? And if I hijack a lot of computers for a spam farm, I can send as much as I like?
Spam is not only defined by bulk. You still need moderators, and that means a process for establishing and maintaining who moderates a particular group. You need people to delete the child porn and the death threats, and that means a process for deletion. (Signed cancels were a pretty good solution in the end). You probably want some means for identifying persistently troublesome users and their sockpuppets from legitimate new users.
You need hosting so that people can effectively access it despite firewalls and on mobile devices. You need a means of paying for that hosting.
Not really. 300 messages a day would only be a few minutes of compute time and could seriously disrupt a large number of small group.
One such technique was a crypto method, similar to what has been recommended in reducing spam in emails. The client has to do some lengthy crypto transform for the message(and NG) they send. On average, it would take .5 sec to generate per message. Small amounts of messages would get through, but spammers would be unable to bulk-send without some sort of supercomputer.
The appropriate algo could be agreed up and updated to increase the amount of time for the message. Even in worst cases, you're utilizing a slow computer for a second or 2. Solves the spam issue.