It's not any of those things and avoiding bad employees certainly is good for the market, it prevents wasteful expenditure.
> Do I really need to explain why putting another persons current job at risk in order to gain a small advantage for yourself is unethical and unprofessional?
To say you're putting somebody's current job at risk is completely hyperbolic. They've already accepted your offer, if they lose their job (how?) and you hire them, you aren't putting them at risk. (And what, you think a past coworker is going to contact an employee's current boss and tell him the employee's going to leave? Yeah right. It's implausible that that would happen in any particular case, not implausible that it happened nonzero times in the universe, but again, you're hiring them anyway. Unless it turns out they're horrible to work with, in which case, the employer and the employer's other employees avoided a lot of needless suffering.)
> It is one thing if you have a team member or good friend who has worked with the candidate in the past and another entirely to go digging into somebodies working history to find old/current colleagues/managers to talk to.
You've put a distinction here but you lack an explanation why one behavior is immoral or unethical. (Specifically for old colleagues/managers.) Yes, you do need to spell it out. Or you could just not bother because who cares.