But with spending money, you can theoretically have more information on the buyer which you can use to help identify and fight spammers/bots & their networks. For example, you can limit one twitter account per credit card or registered user (identified via payment method). If they are found to be spammers, you just kill the account and ban the payment account(s). They can still obviously work around this, but the cost and difficulty for the spammer increases.

I don't know how the financial transactions & stuff work in the background, but the point is that you have more information and more options.