If an account farmer (or anyone) makes a large number of low value posts then they will likely get banned. A well established account will need to make a decent number of low value posts before they are in danger of getting banned. An account farmer could simply make generally interesting posts to get around violating this rule. An account farmer would need to put in a generally small amount of effort to not break this rule.

Yes, someone with a lot of accounts can make a decent amount of money just by posting in signature campaigns.

If you assume that someone can make 0.001BTC per post (which I do not think is unrealistic, and is even maybe low), and over the long run, can make one post every 2 minutes (again, not unrealistic, but the post quality would not be very good), then someone can make roughly $18 per hour just by posting while participating in a signature campaign if they can find enough threads to reply to throughout the day.

When you compare this to making $8-$9 per hour, flipping burgers at McDonalds posting large numbers of posts can be very appealing to a certain group of people.

There are certain policies in place that make it harder on a certain type of spammers, primarily those who post links to spam, and those who use automation to create accounts with the intention of creating a single post (from each account) that is an advertisement. These policies generally give a little bit of value to accounts because higher level accounts due to relaxed posting requirements. The forum also explicitly allows for people to have as many accounts as they want because of things like freedom of speech.

I agree that we have a problem with users making too much 'white noise' making it difficult to have a decent conversation about Bitcoin, however I am not sure how to fix this issue. Over the past couple of years, several solutions have been suggested, however if implemented, they would likely do much more harm then good.