You bought the car. You're free to put whatever fuel in it that you like, but anything other than our own Super Xtreme CarFuel Plus is not supported (and will likely total your vehicle).
A bit more seriously: there are shades of grey. Just because they gave you something for what you paid doesn't mean it was a) fair, nor b) what was (effectively) advertised. Legalese around "you really just bought X" doesn't necessarily put everything in the clear.
i.e. when you buy an Android phone, it seems reasonable for people to expect that to come with a functioning Android-based OS. Just because you can install a different OS on it doesn't mean the default one isn't subject to criticism.
I see it something akin to the smart tv market where you buy the hardware but it also collects and uses data about everything you watch. The data collection aspect allows them to sell you that tv for less than they would if it were a "dumb" tv.
A bit more seriously: there are shades of grey. Just because they gave you something for what you paid doesn't mean it was a) fair, nor b) what was (effectively) advertised. Legalese around "you really just bought X" doesn't necessarily put everything in the clear.
i.e. when you buy an Android phone, it seems reasonable for people to expect that to come with a functioning Android-based OS. Just because you can install a different OS on it doesn't mean the default one isn't subject to criticism.