When your mother buys a printer and AirPrint happens to not work with it, she might ask you or anyone tech-knowledgable to make it work for her.
Since the iPhone has locked you out of doing anything that isn't Apple-certified, your only reply to her will be, buy a new printer. This time, make sure it has AirPrint support on the label.
If the iPhone hadn't been locked down (eg. it's jailbroken), you could easily install additional printer drivers or support.
Yes, buying an iPhone is giving up the freedom to make your new computer do things that you need it to do but aren't certified by the vendor. And yes, consumers do suffer from that. Stop blinding yourself to that. The iPhone would work no different for your mom if there had been a way for techy people to become root. The only difference is, now any techy person can help her. Not just the Apple-certified ones, and not just with Apple-certified solutions.
That is what software freedom is eventually about. It matters to tech people just as much as it does to non-tech people, because it enables them to go to tech people for help. Stallman's formulated four freedoms are simply the rules he figures will guarantee a consumer's freedom to control their own devices, or get help with them from a knowledgable person.
Similarly, in your car analogy, it would be nice if vendors released sufficient documentation publicly so that the car repair person next door who happens to be a really awesome mechanic can help me with my car's issues. Instead, I'm forced to suffer the pain of finding a vendor-certified dealership. That pain is not for the better of me, kindly stop lying to me.