Cloud seeding is done openly on a small scale by individuals or Water Districts. It involves putting heavy metals in clouds in an attempt to cause changes in that cloud. Chemtrails are a vast, well-funded, intentional, environmental contamination operation. The justifications and motivations are probably varied and multi-layered. No doubt the operations are "justified" and compartmentalized by those involved.
Even if "chemtrails" do not exist, any sane person would have to realize that contrails are actively altering our environment, even if unintended. Even a mist of pure water vapor that can cut sunlight and disrupt weather patterns will affect us immediately.