[rwallace]

I can think of a good reason: it's a negative sum game, because willingness to proxy-spam a potential employer has no positive correlation with the ability to do the job; if people start doing this, everyone will be worse off. So if you're an employer and you don't want to end up on the receiving end of this kind of spam every time you hire and under pressure to send it every time one of your friends is looking for a job, you should reject anyone who tries this tactic.