Re: Shouldn't there be a rule to prevent burst-posting or spam such as this?
Moderation is also a "job" that is on a voluntary basis, the only difference is that we have a custom title and dedicated more time to it. 
Of course it's voluntary, but moderators are chosen to do a specific job, and those who don't do it over time probably lose their moderator status, right? In addition, moderators are paid (at least they were before) for what they do, and that is one type of motivation - while ordinary members who post reports generally have practically no motivation (from the forum), not even those badges that have been written about for years.
Back to your statistics; there are reportedly high spam on the gambling board no doubt but going by your percentage 25-30%/100% is below average which means we have way more quality posts on the gambling board overall. If we're being honest, the rate of spam on the gambling board has dropped significantly this year, and the mods are more active now than before.
Honestly, I was too modest with that percentage, I think it's more like 50%, but that's still a matter of personal assessment. Anyway, I think that there is a lot of spam, but that there are too few people who still care about it.