Why would they take a percentage of your salary? You're the recruit. They're providing the business a service (good recruits who can fill a position), so they charge the business. You're providing the recruiter a service (being that good recruit), so they pay you in the form of a signing bonus. Additionally, since the employer pays the recruiter a percentage of one year's salary, the recruiter is incentivized to make that salary as high as possible, so they'll negotiate on your behalf in that regard.