Same reason I don't litter in my back yard. I control what I can control an only that. I'd rather say something and feel like I tried, even if I know nothing will come of it. It makes me feel better about myself, and optimistically might convince a few others to do the same.
I understand your mindset. And if you want to buy organic or whatever else makes you feel good I won't mind one bit... but if it starts costing me money just to have other people feel good I get more than a little annoyed which is why I hate with a passion the constant recycling campaigns that seem to never end, it costs me tax dollars to have those recycle centers set out there because the reality is recycling costs more than not recycling and it only gets done because of government (taxpayer funded) subsidies. There is a reason no business ever popped up on its own to recycle things beyond glass or metal - it doesn't make fiscal sense.
If your opt out plan could be done with no extra cost or reduction in revenue to Disney I would say three cheers for it, because as a Disney stockholder I'm all for Disney's bottom line... but as others have noted it wouldn't be easy and it would likely cost more than it is worth so I don't want any changes to the mass mailings of magic bands.