Big Ag is extremely destructive to the environment, because it doesn't respect existing environments & ecosystems, instead seeing them as a blank canvas for terraforming and imposing industrial processes.
Where are you getting the idea that Big Ag has less environmental impact than a backyard garden???
>They will say stuff like "it's practically free to have all this free food I grew on my lawn", while ignoring the massive amount of labor they put in,
It doesn't require much labor. That's one of the main points — permaculture is about working with nature rather than against it. Instead of tilling and pumping soil full of mined fertilizer, you use the natural processes appropriate for your local environment to support your crops. When properly applied, permaculture is about minimal effort.
>the waste product that they managed to scavenge,
Yes, that's also the point of permaculture: diverting waste streams and closing loops. For example, composting is a great way to make high-quality soil while diverting food waste from the landfill where it would anaerobically rot and generate methane. Permaculture can (and should) be low effort.
>and the infrastructure they pay little to no taxes for.
Huh? Where are you getting the idea that permaculture encourages resource theft and tax avoidance?