Referral Fees

A tight labor market has once again made referral fees a hot topic.  In year past, rewards for employees for passing on the names of their friends and coworkers reached a normal high of $2,000 (there are some rare exceptions), which never quite made sense to me.  Three questions about corporate referral fees for their employees/

1) If a company was willing to pay a fee of $20,000 to a third party staffing firm, why wouldn't they pay more to their employees for finding the same person?  The money could even come out of the staffing budget (the source of the funds may be the problem, now that I think about it).

2) When the friend is placed, are they angry at their friend for taking $2,000 out of their potential salary the way that many candidates think recruiting companies affect salary?

3)  Are there potential discriminatory complaints that can be made from a company that hires people that are referred to them?  Would this be a potential legal issue that has to be vetted by employment counsel?

If anyone has these answers, I'd should like to know.  Maybe I'll ask George.