It isn't about what it is today when life is good. It's about what it can become if life gets hard. If everyone agrees this is a good thing they'd all adopt the poison pill. After all, if the poison pill triggers, the organization is clearly acting against its principles and dissolution is better than continuation.
The fact that they won't, and instead use toothless codes that can be repealed when things change make it clear they don't believe this is an immutable part of them but is a marketing ploy.
Life is hard right now and unionised workers all over the world are fighting to make it better.
How would such a "poison pill" even be enforced? If the majority decides something they can always do it, worst case by creating a new organisation which they then all join.
The US is very racist and such attitudes persist even in the labour movement. Historically, US reformist unions used to exclude non-citizens from membership, which is why the IWW started organising all workers including immigrants. The result of this democratic struggle is that US unions now organise all workers.
You're not losing anything by joining a union. It's possible that you may not gain as much as you'd like because of bigotry where you work, but that can also be fought against.
The fact that they won't, and instead use toothless codes that can be repealed when things change make it clear they don't believe this is an immutable part of them but is a marketing ploy.