A union may protect your rights, but they do ask for some in return - often the ability to work at non-union jobs. The point is, you're still "signing away your rights". You're agreeing that you won't do certain things that are within your right to do, as a condition to employment.
Arbitration is ubiquitous because even at the cost of hundreds of dollars per hour arbitration is a lot cheaper and faster than going to court. And if people don't like the results of arbitration, there's still the option of claiming impartial arbitration and filing a lawsuit anyway. Companies that have arbitration agreements still get sued.
Response to techsupporter, HN isn't letting me make a new comment:
> This is not accurate. Almost every mandatory binding arbitration clause of substance includes wording that the arbitrator's decision is final and cannot be appealed.
There is nothing to stop you from appealing anyway. The fact that you're trying to seek an outcome different from the result of private arbitration does increase the chances that the courts will decide to reject your case. It onus on the plaintiff to prove that arbitration was not done correctly.
There are good observations to be made about the impartiality of arbiters - it's companies that are usually the ones paying for their services, and so it's not hard to image that this creates a conflict of interest. But a company that can force arbitration, and then use an arbiter that had agreed to dismiss all complaints is not correct - the kind of scenario you lay out is exactly the reason why you can sue even after agreeing to arbitration.