Because they can make a little money... See this example of a recruiting ad for professional spammers: freelancer.com/projects/SEO-... (Some forums will even hire people to make posts and create the appearance that the forum is very active: forums.digitalpoint.com/show...
Yes, the vast majority of spammers (and the scammers who send 'dearest' emails) are being paid (in theory) rather than being the originators. It's a job, fair enough. But I often wonder if they pay in the same way as the 'stuff envelopes' type of homeworking...in other words, they don't pay you at all if they can possibly avoid it. I notice the advert you quote states the 'company' won't pay anything until 300 posts have been 'satisfyingly delivered'. All too easy to get out of paying anybody anything with that sort of caveat! :-)
Yes and only 20 cents a post and for the 300 posts they pay60 - make better money walking the streets and checking the gutters for lost coins.
The bad spelling and poor grammar in the ad is a tipoff right away and I question the legitimacy of that ad.