I own a recruiting firm and have been in the business for years.
Simple answer to this:
The recruiting firm needs protection.
Explanation:
Unfortunately a few bad apples spoil the process for everyone. When you place a candidate making $100k on a 20% fee you're looking at a $20k check.
After spending a week scouring job/career boards, leveraging personal referrals and relationships, another week interviewing, and yet another week droning through the background checks/drug screens/reference checks/paperwork/etc. you've spent quite a bit of time/resources to come up with a good candidate.
When you're paying a recruiter, sales people, admin staff, and overhead, that $20k goes quick.
You can't risk calling a candidate and discussing the client only to have them hang up and contact that company directly (or another recruiting firm they've worked with in the past). Most people wouldn't do this however all it takes is one person to take advantage before you require the agreement on the front end from everyone.
I speak from direct personal experience when I say that it can be devastating to spend almost a month on a job, only to find out the client hired a candidate that YOU INTERVIEWED but didn't present because the candidate stopped returning calls and went after the job on their own (or through another agency).
Also - most RTR agreements are client specific. So if we send you to company X, you can only be hired there through our firm. Normally there is a time frame of 3-6 months to prevent the client from trying to bypass the recruiter after being presented with the resume.
Unfortunately, the agreements are necessary to prevent the small number of bad apples out there from hurting your business