There're many payment systems in the market. Some of them have low basic pay and have to make up for it through transactional commission. Some have average pay but their incentives (& disincentives) are also transaction based Some are a mixture of both.
Like all companies, their aim is to make money. Getting someone to fill their customers' vacancies earn them their money and is their priority. All other things are secondary.
Given a choice, it'll be far more better to apply or be hired directly.
by the way, i am not sure what you are asking, but from what i have seen, doesn't mean their pay based on your pay means they will fight for higher pay for you. take for example - you are a person seeking for $8K salary, company wants to give $6K salary. the recruitment consultant has loads of $4K, $5K jobs to clear on their list. obviously quantity > quality of commission in this case. it is many factors.