There's nothing wrong with seeking out companies with strong human rights records or "written policies" as suggested, but ultimately there is no legal contract between you and the company as policies are changed and companies are bought and sold all the time -- again, and perhaps utimately, they can fire you at will.
While is my hope that awareness spreads among more companies that this kind of acceptance thing becomes the norm in business practices but I'm not counting on it as we have seen, companies are quite willing to scrap "norms" if certain "conditions" arises -- we've seen this in the decline of health benefits and more disturbingly, the yanking of pension plans!

Unions on the other hand exists to protect the employee -- they work for you, it is their job to protect you from employers trying to strip you of your job. Its not an adversarial situation, its their job. Company HR officials and union get together all the time to hash things out -- if they can't agree, things are settled with an arbitrator.
While everybody may approach things differently, I prefer to approach an employer from a position of strength -- meaning not only I've talked with my own personal lawyer but also the union's lawyer. I'll know exactly what my employer can or cannot do ahead of time.
Indeed, one large NW TS-friendly retailer has changed ownership a few times and has quite a few employees that have had the SRS, (including 3 at the SAME store) but these employees are breathing easy, not because they still have a TS-friendly employer, but rather, they have a UFCW contract and cannot be fired regardless of who is running the company or company policies.
We can agree to disagree about approaches if you'd like.