LOL our `90 Miata never had an injection cleaning in the 20+ yrs we owned it, same with our 90 LX  5.0 Mustang and our `01 330ci BMW. Now I did run some injector cleaner through them every 4years or so, even tho' I was told it too was a ripoff. 
Methinks you are being subjected to the legendary stealership ripoff.