I don't think this is a huge problem. The value to an employee is in the $10,000s per year, and so its worth it to go through a few spam emails to find the good offers. The value to companies is also $10,000s in saved recruiter fees. So on both sides, to go through 50 spam emails to find one good opportunity is worth it.

And the penalty for being such a spammer is that clients/employees won't want to work with you, and potentially expose you on twitter as being someone to avoid. The risks are high to spam this.