This is bigger than you or any other singular responsible owner. It does not matter whether a portion of the people adopting animals are responsible the majority are not. You are in the minority.
People barely provide sufficient medical care and keep their dogs in outdoor kennels or locked inside all day. The average dog is poorly trained and receives minimal exercise and stimulation, allowed to become morbidly obese and often suffers from chronic ear infections (likely tied to the cheap grain-heavy diet). If you truly do not think that this is the life of the average American dog then you really need to step back and look at the big picture. Maybe in your animal-friendly sliver of the country you don't see the reality but that does not mean it isn't there. Americans are not responsible with their animals.
Even with low-cost spay and neuter clinics people cannot be bothered to get their animals to a clinic. Even free spaying and neutering is not enough to tempt many people to get their butts off a couch and to a mobile clinic.
Knowing this, why on earth would we arm these people with the knowledge that there are health benefits from spaying and neutering later? Where are we going to get the funding to provide for the extra animals irresponsibly created? Where are we going to get the resources to police people down the road and ensure they spayed and neutered post adoption? How are we going to handle people who will not comply?
In terms of policy making, the costs of these potential health issues are far less than the potential repercussions from allowing people to adopt intact animals and informing them about the potential health benefits.
I would never support a rescue that allowed animals to be adopted out intact unless they had the funding and legal support to reclaim that animal if proof of neutering was not produced by a set date and the likelihood of that seems slim.
We are the minority.
The apathy that most people have towards their "loved" pets is truly staggering. 6 years of experience working in vets office taught me that ignorance and neglect is not limited to geographical area, SES, education, gender, age, race, etc. It is pervasive and deeply rooted in America. Until people change the way they approach animal ownership, early spaying and neutering is the least of our concerns.
*steps off soapbox to take intact dog to the park*