I agree on the problem from used tires. In MT, tire shops usually seem to charge a disposal fee. Last I checked, it was $6 / tire. Like you have observed, some just use roadside ditches if they are trying to be somewhat responsible. Others just drop in the roadway. When the landfill charges by the load, ditches seem to attract used appliances, furniture, & etc. Not sure what the solution is and I know I'm only one guy, but I try to be responsible and not add to the problem with my actions. Don't think it can hurt. I was somewhat surprised to see the Hankook guys using recycled for some of the tire. However; they are in Hungary so probably not helping the used tire situation in USA.
As for additional fees when buying tires; AZ seems to be much different than MT. If I'd bought these tires from WalMart, Amazon, or online guys, that would have just got the tires for around $350 each and I'd still be on the hook for all other charges. Reputable shop where I bought these just quoted OTD price of $1410. No sales tax, mounting & balance, disposal fee or nitrogen charge. 
I'm a bit curious what a $150 car tire would end up costing you when all extras are added on?