How do you stop signature spam when paid signatures are allowed? There are money-making forums and guides that encourage registering on here and posting simply as a way to earn an income online. While that $3 a day or whatever signature posters get paid may seem insignificant to some, it is pretty enticing to people in some parts of the world (eg. official minimum wage throughout India varies from $2.40 to $6.35 per day; and there are many places with lower wages than that). Multiply that $3 by several accounts, and they've got a a pretty good thing going for them.
You need to remember than in poor countries, there's probably a £100 startup fee for Internet connection. Mainly used for cabling and routers. There's then a £20 fee per month at least in most countries to maintain a connection. The money earned is a bit worthless, however, it all adds up and is probably seen by some as a good way to greater decentralise the currency.
You can get a 3G data connection with a few gigs for less than $5 a month in many countries. More people in India have access to smart phones than toilets. Maybe some of the signature spammers can shed some light as to why they spend time here making incoherent posts that no one wants to read instead of exploring other sources of income.