I'm not sure I would say most. They are in the business of making money and selling you services. It is your job as a consumer to know if you really need an injector flush or not.
The bottom line is you can put it back on them by saying, if you do this repair and it doesn't fix my problem, you want them to put your car back to the way it was when you came it and only pay the labor for the original job.
You should not have to pay for their guessing. If they are not willing to stand by their diagnosis with their money, take your business elsewhere.
For many of these other services, such as motorvac's and transmission flushes and so-forth, educate yourself and decide if you really need them or not.
A consumer is only ripped off if they say yes to something they don't need. So make sure you are informed and use your power to say no if you are uncomfortable with what the service writer is suggesting.
If they deliver work they promised to deliver, that you said yes to doing, you didn't get ripped off. You may have been sold something you don't need, but you said yes.